I hope that Napoli really came out of the crisis, but it's too early to say about it - they won only one serious match after a long unsuccessful streak. But in any case, this meeting will take place only in mid-February - by that time a lot may have changed, but I suppose that both clubs will rather be busy with business inside the home championships and will not focus on the Europa League.
In this pair I believe more in going further Barcelona provided they have a couple of good transfers. Barcelona is able to add in the course of the season as they have a good coach Xavi whose handwriting is still very poorly visible since there are no worthy players for his game.
Napoli is now fighting for the championship and if such a fight will continue until the match with Barça then I don't think that priority will be given to this match.
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.