The transfers of good players are just potential opportunities (we don't know if they will take place). While the real situation now is this: Napoli played excellently at the beginning of this season and is only now experiencing a failure. Barcelona has been playing poorly all season and Xavi hasn't changed anything. I see that the bookmakers quote Barcelona's odds higher than Napoli, but most likely because of the "name", and not because of the real balance of power:
UEFA Europa League 2021/22 - FC Barcelona reach Round of 16?
Yes 1,69
No 2,05
I agree with that Barcelona's name making an impact of the odds and not the real power of their football. They are really not doing so good this year. Europa League is of course different but in La Liga they only managed to get 8 wins out of 20 games this season. And I would expect they will not go too far on the Europa League as well. They are just not the old Barcelona, although the reputation is still there. Therefore the odds, I think.
The fact that the bookmakers are based on the opinion of the crowd, and the crowd believes in the names very well in fact. For example, I see no reason why a bet on Barcelona's rivals (in this case Napoli) would not be profitable. I understand that the result can be anything, but if you take the distance, then Napoli is clearly underestimated in this pair.