Lukaku has always been a great footballer, the problem he had was that he never fit in Chelsea system so he had to be loaned out constantly when he was there, finally he figured out a way to permanently make a move. He also got injured a while back, which made him a bit slower, but he is still talented, maybe he is not as fast as he used to be, but his speed was never the intended attribute that teams looked for him, his positioning, his heading, his shooting and technical abilities were all the things that made him who he is.
On the very first game, we saw how important those are, he helped his team make a comeback on the last minutes, and that's why I think it's important for Conte to get who he wanted, and he basically got rid of Osimhen.
Lukaku is a good player, I just think he has to be in the right team to show his best ability, the 2020/2021 season was the last season he could have the best performance with Inter Milan. The next season he went to Chelsea and that was the season where his performance declined drastically and he was only there for 1 season after that he was loaned to another team.
Indeed, a player sometimes has to find the right team to be able to show their best abilities and when they don't find it, it will greatly affect their form of play. The style of play of a league is also sometimes a factor in where they will succeed and fail, because there are leagues that suit their style of play and some that don't, and for now I think the English League is not suitable for Lukaku.
We can see Lukaku's statistics every season here
Transfermarkt.