How sure are you that Napoli would beat Barcelona in their next UCL fixture? Even with Osimhen in the game, I think the odds would be against Napoli.
Although the Afcon would require Osimhen's presence because the super Eagles don't have an established striker like him, unless we call Iwobi who is doing well at fullham, then he would be unavailable for the rest of the UCL matches or la Liga matches respectively.
It seems like no one is sure that Napoli will be able to beat Barcelona apart from Napoli themselves because of course they have to maintain optimism to raise their motivation, especially in the first leg, but of course, Napoli also realizes how much difficulty they are facing in this match even though the opportunity is still there but look at how their current condition means it is a tough thing to beat Barcelona.
Barcelona is actually not as superior as many people see, they currently often have difficulty beating teams that consider can easily they overcome, especially in Laliga, several wins in local competitions seem like a bit of luck, so if Mazzari can implement the right strategy then Barcelona will also find it difficult to beat them in the first match, as long as the result is not a defeat for Napoli or a draw then Napoli has a chance of advancing to the quarter-finals, but as you said without Osimhen maybe Napoli's big problem is scoring goals.
In fact, these two clubs have similarities in terms of bad things, such as Napoli, which has difficulty getting the right strategy because Mazzari is still too new as Napoli's coach and it takes a long time to be able to read and provide the right strategy for Napoli because coaches are also very influential on the future of a club.
Meanwhile, Barcelona still has the same financial problems which prevent Xavi from being able to bring in more quality players and Xavi is forced to try to strengthen his players only with existing players.
But in terms of experience, it is actually very influential for the club to be able to continue its journey and qualify for the next round and Barca advantage only lies in experience and Napoli superiority lies in their truly quality players.