Lukaku might be can recognized as flop player in Chelsea because on that team Lukaku never able to playing consistent but in my opinion when loaned to Inter Milan and Roma Lukaku performance is quite impressive even in Roma he is top scorer of this team on Europa league so what i want to says is Lukaku is suitable to playing in Serie A and everytime he was playing at this competition although with different clubs but his performance is quite stable so might be this is the main factor why Conte want him instead of Osimhen
Lukaku has just make his debut with Napoli and he can scored and bringing Napoli to beat Parma and i think this is good start for him especially after Osimhen didn't including to Napoli squad of this season and loaned to Galatasaray then Napoli hasn't have sharp striker and probably Lukaku can solved the attacking lines of Napoli
We had expected it before, when Napoli have 2 players like Lukaku and Osimhen then they have to choose one of them, of course Lukaku is the best choice because his salary is lower and his performance is also impressive. We will see how far Lukaku can play with Napoli who have a good record under the same coach, as you said Lukaku has made a good start by scoring a goal in the last minute and that also proves that Napoli made the right decision. Now it's Osimhen's turn to prove Napoli made a mistake by wasting him.
Honestly I feel quite sad when I see these two strikers changing each other in the match because both are quite great in my opinion and have a good track record, when one is useless and sees only a few weaknesses for the coach then of course it will not be used even though their abilities are almost the same.
But on the other hand there is also a good thing, where these two strikers will be like competing, both must give their best so that they can be trusted to play more on the field, of course if there is no further conflict just as motivation for one another Napoli should be able to get their victory back well with Lukaku and Osimhen.