Napoli really have a bigger level of performance than Barcelona now. And it must push Barcelona to work harder to get well-prepared for the matchup in the Europa League. They have no luxury to underestimate any of their opponents now. They are already struggling and things will get worse for them if they don't do something about it immediately. They need to improve their results in their domestic league firstly.
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.