This narrative is completely false and has been exaggerated by his fanboys just because he scored some goals in a row in like 4 league games or something. Which was much after he came back. He was part of the problem for far longer than he was part of any solution (in Serie A, because he was almost useless in CL)
We managed to bounce when we played Benfica and drew Napoli or Milan for the SF, which meant that Inzaghi started to play for two fronts rather than be in complete disarray and not sure what to choose from. Lukaku returning merely coincided with that. Unless you want to credit Joaquin Correa as well, because him starting also changed this
We got through the groups with almost zero input by Lukaku (you can highlight a 4-0 goal vs Viktoria if you wish), by beating Barcelona and almost beating them away, by scoring 3 there. Not even "healthy" Lukaku managed to help us do anything near that against Barcelona B at home, let alone their first team away...
Apart from the Porto goal which was rather lucky (double effort, goalkeeper didn't expect the 2nd shot right at him) but still it is what counted for the tie, his presence wasn't really significant. You can give him the PK, but we only missed one last season iirc.
If he was considered so valuable as people say, he'd be playing in CL which had been our priority by that time. He had a total of 120 minutes in 5 KO matches, which is like slightly over 25 minutes total if you add extra time which isn't part of these statistics. A 65th minute sub in CL at best.
When did he become a starter in Serie A again?
Mid to end of February.
Let's see the schedule:
13/2 Sampdoria away. His first start since forever against the worst team in the league. Final score against the worst defense? 0-0
18/2 Udinese at home. Started and got his first goal in a long while and like his 3rd goal of the season. 3-1 victory where he scored the opener. The sad part is that we started winning only when he got subbed for Lautaro.
22/2 Porto at home: 1-0, Lukaku scored. As a sub
26/2 Bologna away. Started. We lost (conceded after he was subbed out)
5/3 Lecce at home. 2-0 win, came on as a sub after the score was settled and didn't do his usual stat padding.
10/3 Spezia away. Started and played the entire game in a 2-1 loss. Scored our goal with a PK.
14/3 Porto away. Played as a sub, zero contribution. If anything, was rather useless in counter attacks which was why he was slotted in.
18/3 Juventus at home. Played as a starter in another derby home loss.
1/4 Fiorentina at home. Another start, another goalless loss. Played the entire game once again.
4/4 Juventus away. Coppa semi, scores an equalizing last minute PK as a sub. Well done.
7/4 Salernitana away. 1-1 draw in a game he started. At least he got credited with an assist with a fluke header that was intended for someone else
11/4 Benfica away. Came on as a sub and did what he needed to with the penalty to settle a vital 2-0 away win. Anonymous apart from that.
15/4 Monza at home. Lost in a game he played the entire game [at home] without scoring once more!
19/4 Benfica at home. 3-3 thriller after a casual 3-1 lead for the team, with Lukaku coming on for the last 14 minutes. Coincidentally, that was at 2-1, which became 3-1 almost instantly with Correa and then we suddenly collapsed and conceded 2 in 5 minutes.
23/4 Empoli away. Finally, team morale is high as we are CL semi-finalists. And yours truly finally shows us, scores 2 in an away win.
26/4 Juventus at home. Came on as a late sub, all we had to do was hold on an early 1-0 lead.
30/4 Lazio at home. A win in a big game after a while. 3-1 with Lukaku actually contributing, but only after Lautaro, Hakan and Gosens came to the game.
3/5 Verona away. A rather difficult exit to Verona who were chasing points to stay up, yet we beat them easily 6-0. Lukaku did not feature one second in this game.
6/5 Roma away. Another big win in short space for the team. Lukaku started and scored. This is where he starts his scoring streak in the league.
10/5 Milan "away". Team wins emphatically but can only keep it to 2-0. Lukaku enters in the 70th minute, relatively anonymous.
13/5 Sassuolo at home. Possibly Lukaku's best game in the season with our first and last goal in a 4-2 victory.
21/5 Napoli away. Lukaku gets his 4th in 3 Serie A games with an equalizer that wouldn't hold. In a rather bad outing for the team.
24/5 Fiorentina Coppa final. Came as a sub with 35 minutes to go. Score was already settled at 2-1 from first half.
27/5 Atalanta at home. Scored the opener to continue his league scoring streak (5 in 4 games)
3/6 Torino away. We won 1-0 and he ended his scoring streak. Gets credited with an assist simply for being the last player to lay a pass to the scorer who hit a wonderful strike from far.
10/6 Manchester City CL final. Well, it is what is it...
So when Lukaku came back - and I'm going to include the three games he featured as a sub before the start of that list (Milan derby win, Cremonese win and Empoli loss and the Atalanta win where he started, right before the derby) we have:
Overall:
18 wins
5 draws
8 losses
In Serie A:
11 wins
2 draws
7 losses
As a starter:
8 wins (one in Coppa Italia)
2 draws
6 losses [including 3 in a row and a 5 winless game streak!]
With Lukaku not starting or staying benched:
8 wins
3 draws
2 losses
Games he did not start include CL quarters, semis and final, Coppa semis vs Juventus and final. So it's not as if we saved him for the tough ones...
So by looking at this, when Lukaku started, we had a much worse record than when he did not.
We actually won less than 50% of our Serie A games when he returned to a starting role.
For someone earning 50% more than your best striker, his output was far less than half of that. Complete waste of money, and apart from the goal vs Porto, a late PK vs Juventus in the Coppa semi (1st leg) and a scoring streak in May, this was a completely forgettable season. Leaving the CL final memes aside and all the media garbage that follows.
So no, I think the narrative that he somehow resurrected our season has to get dropped by every single member at FIF. It's offensive to the players that actually did contribute to the run. Lukaku had a good 20 day period in the league in the final stages which is why a lot of people sweetened to him during that time and why some got persuaded that him coming back on a full transfer would be prudent. But that's really a very small sample out of a very poor situation that gets magnified whilst everything else gets ignored and falsified.
I'll repeat that Lukaku's return hasn't provided any boost. If anything, the numbers show that when he started, we struggled even more. Now I'd like to see the arguments that it was because Correa, Gosens, Bellanova and a few others were playing as well... Which kinda ruins the narrative in place as well and just reinforces that Lukaku has different treatment here for whatever reason people adore him.
Saying all that, he'd be a good addition for Roma on a cheap loan, as long as Mourinho knows how to handle him and actually wants to do that. But aside from that, him returning to the league isn't some coup.