Well everyone is excited about Lukaku back in Chelsea but I have doubts over his arrival. He sure had great season in Serie A but he wasn't that good in United. Yeah he scored but his stats in Serie A is far more better than PL. Maybe its the league level or maybe he has now developed? but nothing can't be said for sure.
tbf, the managers didn't utilize his full potential. Mou's gameplan was all over the place and Ole was coaching a top team for the first time. IMO Lukaku fits perfectly in Tuchel's tactics which focus a lot on counter attacks, a similar style to Belgium NT and Conte's Inter Milan (sometimes, tho)
Chelsea are taking a big risk Lukaku did not work out for them before and it is not certain that Lukaku is a better player now than he was then because the league is different.
10 years ago he was a promising start. Now he's a world class striker and among the best in his position.