It's not really weak like you said above. Given that Napoli was competing against a team as strong as Real Madrid, it would be inappropriate to criticize their performance as weak. I have opposite opinion about that and it feels like napoli is declining after it has done all of matches in the group stage and qualify for the next stage. Napoli is not as good as last season but it can still compete to face barcelona soon. To put all of the players into the defensive line, the only thing that needs to be done by Mazzarri is to make the necessary adjustments. The club shall take counter attack as opportunity to beat barcelona.
Barcelona was a club that is focus into how to build more ball possession. If Napoli focuses on playing with a strong defensive strategy against Barcelona and utilizes effective parking of the bus, they can significantly increase their chances of beating them easily. Napoli's group considerably placed at the medium group.
It has real madrid as the main competitior and do you forget that if barcelona got easy group compared to the napoli? The way to beat barcelona to go through the fully defensive line like parking bus strategy.
It will be making barceloan frustrated with it. Barcelona is going to lose its concentration when the club was always trying to pressuring napoli.
Between Napoli and Barcelona currently, there's no much difference because both teams have flaws they're battling with. For Barcelona, they're not so grounded yet as there's a bit of shakiness and inconsistencies in their usual performances before now. They're now at a low leveled performance phase than they are normally known for.
For Napoli, the team just got a new boss and so Mazarri is trying hard to implement strategies that best fits his team and would give them the required results. There are still weeks to the round of 16 games, so it's more chance given to both teams to improve themselves in their various league games and then be better prepared for the encounter in February. Judging both teams, they've got same odds of qualification based on the current teams' technicality and definitely the players in the team are eager for a win.