yes, everything will be fine, but without Osimhen it will definitely affect the performance of the Napoli team little, even though it looks like there are no problems In fact, we will see all of that in the UCL match and achievements in series A later. If in series A, Napoli is able to get back to the top of the standings in series A, I would say Garcia was successful in bringing Napoli to shine without Osimhen, but if they fail in the UCL, maybe that fine because its too early for Napoli to get trophy in the UCL.
No doubts without osimhen they will be fine after all before he came they have been fine, there are also record of teams having big players leaving and at the end of the day they were fine such as Juventus and Ronaldo, Messi and PSG etc, but we can't denied the fact that the time they spent in various team gave these team a good face lift till they left, and the performance of the team after their leave affected the team.
Definitely they will be greatly affected in their UCL performance if osimhen leaves, if they managed to make it pass the group stage, I doubt if they will make it pass the knock out stage, the presence of osimhen in that team has a lot of impact on them as when they are facing other team they get this consciousness that there's a striker who would give them a little of tough time, but in his absence this fear in other team may not be there, and for the first few time, the effects of his absence will be felt greatly.