Barcelona and Napoli now have their own problems. For the decline I see Napoli as having the worse decline, we can see that in Serie A. To be honest, I always use the domestic league as a measure, even though it's a different competition, but in the domestic league they have a schedule every week, so we can look at them and judge every week, whether they have decreased or improved.
I agree with you that it won't be easy to predict this match, and it's possible for them to beat each other and it will be their goal productivity that decides it. Both Napoli and Barcelona will have to watch out for each other.
I used to believe that both clubs are on the same level currently when we talk about current form, but recently I saw a tweet on X by Fabrizio that said Inter has agreed on a deal to sign Zielinski from Napoli. I don't know if that deal is for the end of the season or during the winter transfer window. If it's for the winter transfer window then this is bad news for Napoli.
Zielinski is a very crucial part of that team. He's arguably their best attacking midfielder and without him, I don't see how Napoli's to cope against Barcelona. If Napoli will get a replacement then there's no problem, but leaving that spot unfilled will leave a void in that team.
Barcelona is a team that plays midfield football so your midfield has to be very good if you want to win against them.
I do not know the financial situation of Napoli, but other financial problems, I don't see a reason why a club would sell a player like Zielinski halfway through the season. Napoli needs to start scouting for a replacement for Zielinski because their problems will increase if he leaves.