Barcelona is currently not in a good situation, otherwise, a team like Napoli could never beat Barcelona with the performance they had during the last season with their coach Xavi, but since there are many injured players in this team and there are many players who left the team, I think this can be hard as you said to predict this match.
I think Napoli can have a good chance to score a goal in this game if they have Osimhen in the good form because the defenders of Barcelona have not performed well recently.
Injuries can significantly weaken current Barcelona that makes them vulnerable even against teams they might have historically outperformed. Under normal circumstances, Barcelona, with their historical prowess and talent, would not be easily defeated by a team like Napoli. However, the current circumstances have altered the competitive landscape. But Napoli is not at their best too, even though they still have Osimhen who will terrorize the vulnerability of Barcelona's defenders. Barcelona's defense has become a big issue this season suggesting that they will struggle against good attackers.
Both Barcelona and Napoli not being good this season implies a certain level of uncertainty and difficulty in predicting the outcome of the match. This introduces an element of unpredictability, making it hard to confidently assert the outcome of the game. There is a rationale argument for Barcelona's recent struggles, but Napoli's decline makes it easier for Barcelona.