Do not think anybody underrates Napoli. They are simply a very strong team, like easily would rate them around top 10 world. At this level you could loose any game, even during great season.
My optimism about the season, including the match against Napoli, comes more from overall trends than from the rather ambiguous performance against United:
- few players already looks much better (Haaland, Foden, Khusanov, Doku),
- Rodri is starting to play quite a lot of minutes regularly. Sure, he’s far from his best form, but it’s still an early stage. It should only get better,
- new players are really promising, pretty much all of them showed glimpses of top quality: Marmoush, Khusanov, Cherki, Reijnders, Ait Nouri, Nico too is useful,
- Bobb is back, still way to go, but he has magical feet and this kind of special quality,
- Donnarumma is not only promising, but looks like a guarantee for few additional points and wins.