I think it comes real down to whether or not the Barcelona players can get used to the fact that they are playing Europa League and not Champions League. Especially players like Jordi Alba or Pique may have a hard time to motivate themselves to give their very best as it is a brand new situation for them. Maybe it is even good that their opponent is Napoli so it doesn't feel that bad.
As right now Barcelona is struggling badly in La Liga they need to go ahead strongly with this because if their players give their best then surely they can win this title and having spot in next season's Champion League very easily otherwise they need to do some very hard task for having spot in Champions League next year with currently staying in 7th rank with chance of dropping at 8th because their next match is against inform Sevilla which already beat Atlético Madrid at home and now can beat them at home without any serious trouble.
But, no one can give his prediction about these matches as Napoli is also having very good form after losing few points now they are also back on winning track and can beat any good team on their day.