This time last year, Romelu Lukaku signed for Chelsea for a record fee of £97.5 million.

Fast forward a year and it looks like he's heading back to Inter, which is the club he joined from.

And the general consensus seems to be that he's had a disappointing time at Chelsea.

And in this video, I'm going to ask what went wrong for Lukaku at Chelsea?

So on the board in front of me, I've got two pizza charts from the last season at Chelsea and the season before Inter Romelu Lukaku.

Now the temptation when we have these pizza charts in front of us is to look at the colours and think there's much more colour on this chart than this chart.

So he must have been doing much better this season than he was in that season.

Now, to an extent that's true, but we also need to take into account the fact that he's playing very different roles for both of these teams.

So for Inter, he's playing as part of a strike partnership with Lautaro Martinez.

When he moves to Chelsea, he's suddenly playing in a front three.

He's expected to do lone striker things a lot more.

So when we look at the pizza chart in front of us, then, if we look at the areas where he's doing things that we would expect a lone striker to do, actually there's a lot of portability between the two of them.

Receptions in the opposition box is basically the same, shot volume slightly down at Chelsea than it is at Inter.

But in terms of the things that you want your striker to do, get in the box and take shots pretty similar on both seasons.

The area where we start seeing a drop off is in these areas.

So xG from shot creation, xG from ball progression and these metrics are just showing you the generation of chances that he's making.

So he's making far fewer chances in terms of xG for Chelsea than he is for Inter.

And this follows through then into the possession side of the game.

So progressive passing is down his carry and dribble volume is down his link up play volume is down as well.

So that suggests that moving from a two striker system to a one striker system has meant that he's not doing as much link up stuff.

He's doing more striker stuff, getting into the box, scoring goals.

Another thing that we need to think about when we're thinking about how successful Lukaku was at Chelsea was, how good Chelsea were themselves.

Now I've got a rolling ten game average xG and xGA chart here.

So that basically shows the expected goals generated over a ten game rolling sample and then the expected goals conceded over a ten game rolling sample.

And as we would expect, Chelsea are a good side so they're generating far more chances than that conceding.

But interestingly it seems as though after around Gameweek 20, we start seeing Chelsea dropping off a little bit and then towards the end of the season there they're climbing up as well.

Interestingly, Lukaku's best period is probably at the beginning of the season when Chelsea are doing good stuff and he actually has a couple of games towards the end of the season where he picks up a few goals as well.

So actually the areas where Lukaku is playing well are also the areas where Chelsea is playing well as well.

So it's worth keeping in mind that that's another layer of context when we are considering whether or not Lukaku succeeded at Chelsea.

Now normally at this point, I would want to dive into the tactical side of things and look at the difference between a two striker system and a one striker system.

But I think we do need to add another layer of context because actually Lukaku season was very disjointed.

So in the board in front of me I've got just a rundown of the first half of the season in particular and it just shows you how disjointed the season was.

So these are the first seven games that Lukaku plays in the season.

He actually plays quite well.

He plays all of the minutes available to him until he gets injured right at the end of this, get some goals and these are teams who are pretty good teams.

So you've got Arsenal in there, Liverpool in there, Man City and then Spurs in there and Lukaku has probably his best spell of the season here.

He actually puts up the highest expected goals and expected assist per 90 numbers over this period than he does over the rest of the season quite considerably.

So Romelu Lukaku actually starts out quite well.

Now he gets this injury at Brentford and as we can see, misses the next four games.

And after that he's attempting to come back so much fewer minutes here.

And during this period of rehabilitation, he records somewhere around this time that infamous interview that he gives with Sky TV Italia.

It isn't released until later on, but it is recorded around here.

He tries to come back but then unfortunately he's hit with COVID.

So that means he has another setback.

So two games missed here.

And then after that he has these two games, firstly against Aston Villa and then against Brighton, where he scores in both games and everyone thinks that he's about to come good.

And then, bang, if this infamous interview is released and from that point onwards then his season sort of tails off a little bit, he does come back at the end of the season as a couple of goals against Wolves and a goal against Leeds.

And that's so that point on the chart that we saw before where Chelsea are coming good again but very much disjointed season.

And so I think the context then has to be taken into account.

Let's move on to the tactical side of things, because there seems to be urban myths about why Lukaku didn't work in the Premier League.

The idea being that Serie A is a very specific type of league in terms of its playstyle and that Lukaku was allowed a huge amount of space to run into to attack and the Premier League is much more solid defensively and there wasn't space there for him to do that.

But I actually find this fairly unconvincing explanation for why the Lukaku didn't work.

I've got a couple of screenshots here which just show a few games against teams who you might think of as being weaker teams.

And just to give you a sense of the amount of space that Lukaku is being given, especially in transition.

So here's a game against Southampton, this is Lukaku here, he's got half a pitch of space to run into.

So one defender in front of him, Callum Hudson-Odoi is running into this space.

That's a transitional moment that you would expect him to eat up.

Here we've got Brentford, another team that a lot of people think of as being quite defensive.

And again, you can see Lukaku with space to run into, particularly on the right hand side here with the ball being carried through.

Leeds, obviously a team that people think of as being particularly defensive, but again Lukaku finding space there on the left with Havertz carrying the ball through again plenty of space for him to attack.

Finally Watford, another team at the bottom of the table.

But again you can see Lukaku finding space to run into and attack there.

So the idea that Lukaku failed in the Premier League because there wasn't space to attack is, I think, unconvincing.

Now I've got some data to show this.

So on this chart in front of me from The Analyst, we've got a team style comparison in terms of the way that teams attack.

So the most important metric for us to look at here is direct speed.

So this is the speed at which teams are attacking.

And as you can see, we got Inter here, Chelsea here and actually pretty much on exactly the same level.

So when those two teams are attacking, they're attacking at the same speed in terms of passes per sequence at the bottom here, again, there is a slight difference here.

It's about half a pass per sequence.

So Chelsea may be being a little bit more ponderous in terms of their attacking play, but in many respects, Inter and Chelsea, when they're going forward, very, very similar.

And so again, just another level of data which suggests that maybe Lukaku wasn't struggling because of the playstyle necessarily of Chelsea.

And so that brings us on to the tactical differences between playing in a two striker partnership or as a lone striker.

And I've got a heat map in front of me that Maram has made for me.

This is great because it shows you the difference between Inter and Chelsea.

So the red areas are where Lukaku is receiving the ball when players pass it to him.

As you can see Inter, he's playing in a strike partnership alongside Lautaro Martinez.

He's playing on the left, so it makes sense that he's receiving the ball on the right hand side a lot.

I can see there's a little bit of red here in terms of when he's getting into the box, so he is doing some lone striker stuff.

There's also a little red area here which actually you can't see on the Chelsea viz, which means that he's dropping a little bit deeper on this side as well and we'll see that in a little moment.

Moving over to Chelsea then it's very different.

Obviously, there is still that right hand bias that we see there, but we're seeing a lot more receiving of passes in the central spaces in the penalty area and a little bit more on the left hand side as well.

So we can already see that Lukaku is receiving the ball in very different places when he's playing for Inter to when he's playing at Chelsea.

So now let's take a moment to see how Lukaku fits into these two different systems.

So I've got Inter on the board here, we all know they play in a 3-5-2.

This was an Antonio Conte team, so we know exactly what to expect from them.

Conte is going to want his team to build up in wide areas with these automatic passes and finding space down here makes all the sense in the world that Lukaku would drop into the right half space, pick up the ball to feet, and then he has a few options available to him so he can either play that pass down here so that Hakimi can run onto it or he can play the pass into his team-mates here attack more centrally.

So he's picking the ball up to feet.

He can also pick the ball up and run.

He's very good ball carrier as well.

And so that's how he's being used at Inter.

So let's have a look now how he played for Chelsea.

So here's Chelsea's 3-4-3.

Obviously no strike partnership here.

You've got two wide forwards, although they are going to play quite narrow here.

And the first thing that we're going to see Lukaku doing a lot for Chelsea is dropping deep here into this space here and then having these two guys run beyond him into the space and he can do one of two things actually.

The ball often comes from Mendy, so it's coming in the air.

He can either chest it down to one of these two or he can play the flick on for the Mount or Havertz to take the ball up and run into space.

Interestingly enough, he's receiving the ball in the air quite a bit.

So that's why on his pizza chart you can see his aerial numbers were much higher.

The other thing that Lukaku is doing for Chelsea that is quite different to what Inter were doing with him, is that while Chelsea are still getting their wing back into this area and they're going to attempt to play these sorts of crosses into the box for him to attack rather than Lukaku being involved in the build up play closer to the ball.

Actually what we're going to see is him dropping off the back of the centre-backs here and coming around into this area to attack the ball.

Now that's what you would expect a lone striker to do, but it means again, his build up play is just reduced and this is one of the reasons why Inter suits Lukaku better than Chelsea because for Inter he's receiving the ball to feet.

When he receives the ball to feet, it allows him to do something that he's really elite and that is ball carrying.

Now I've got a couple of viz here again from The Analyst in front of me and this shows you all of the chances that Lukaku created for Inter by carrying the ball forward, receiving it deep and carrying the ball forward.

So you see plenty of examples here of him picking the ball up.

On the side here we've got shots and goals, so you've got plenty of shots in the box.

We've got four goals here.

So he's picking the ball up, carrying it forward, taking a shot and scoring.

On this side we've got chances created for team-mates and then assists.

So again, plenty of examples here and then five assists that come from him picking the ball up, carrying it forward and then laying it off.

So if you look at the same viz, but for Chelsea, it's just completely different.

So you can see there's barely any examples of him picking the ball up, carrying it forward and if we go on to the side here.

No goals scored by him and no goals scored by his team-mates by picking the ball up and carrying it forward.

So that's clearly one area that he was missing out on in his time at Chelsea that he really prospered from at Inter.

So when answering the question what went wrong for Lukaku at Chelsea, there's a lot of things for us to consider.

On the one hand, he probably wasn't as bad as a lot of people think, and if you consider him as a lone striker, he did a lot of the things that Thomas Tuchel would have wanted, but it was also a disjointed season.

He got injury, he got COVID.

There was that Sky Italia interview as well, which didn't help.

And as we've seen, there are also some tactical differences between the two teams that didn't help him fit in at Chelsea either.

And that just goes to show the fine line that exists between success and failure at the very top level of football.

If you'd like this video, please consider subscribing to the channel.

The Athletic brings you the best sports journalism in the world and a personalised experience connecting you with the stories and teams that you care about the most.

There's coverage of 13 sports, plus direct access to world class journalists through live Q&A discussions and podcasts.

Not to mention, it's all ad free.

And you can try now for free for 30 days by clicking the link in the description.