Even though Napoli happens to play against Manchester City or Bayern Munich as you talked about, Napoli has got a very good team this season and they can stand and beat any big tram if they should come against them right now. Napoli team has already won the Italian League so good a thing they won't been under pressure or suffer much injury crises. The favourite in the Champions League is Napoli and there's nothing stopping them from winning it.
It's better to talk about what already drawn. Ac milan was easy team that can be beaten by napoli. People may think that if this is UCL and ac milan will be doing pretty well compared with local league but i think that the result will always be the same. Napoli already secured scudetto but it doesn't mean if napoli would able winning the trophy easily without any problem. napoli was among the favorites but it doesn't mean people are only picking napoli as the most favorite team right now.