With the way Barcelona plays, it would be a miracle for them to make it to the semi-finals of the champions. They do not play well at all. They concede too many goals. How can a team like Barcelona be trailing 2-0 to Madrid in under 10 minutes? A defense like that doesn't go far in the Champions League.
The only chance they have against Napoli is that Napoli is also in a very bad phase of their own.
Barcelona's problems are not very big IMO me. All they have to do is stop conceding goals and start scoring goals. That's all they need to figure out how to do.
If Xavi doesn't win any trophy this season, I believe he should be sacked. He has done so well for Barcelona by taking them out of where Koeman put them, but I don't think he's the man to make them a fearsome side in Europe. They need someone else for that.
A miracle might not be the right word because last season we saw how much the drawings can do for teams that are not playing that well, but they are lucky enough to play against some of the "weaker" opponents in the Champions League.
I completely agree that it would be unexpected if Barcelona shows up in the semifinals of the Champions League. However, Barcelona could get lucky, win the game game against Napoli (or the aggregate of the two games) and then get Dortmund/Eindhoven or PSG/Sociedad or Porto/Arsenal. I think against these teams they would at least have a chance to win the quarterfinals, which then would make them reach the semifinals. It would still not be a miracle, but it would definitely be one if they face Manchester City/Real Madrid/Bayern Munich.