I am not too optimistic about this change of coach. Napoli has never been this top team that can perform like for several years like they did last season. Lots of teams have years when they play extraordinary football and seem to be unstoppable. But those that don't do that on a consistent level often experience a decline in their performance in the season after and this is what I thought would also happen with Napoli after this insane first half last season. It was never possible for them to top that and getting close to it. Everything worked smoothly for them, every result was perfect and every game that could have ended with a draw eventually ended with a win for them. Now they are taking actions while I believe that the also failed to properly classify what they have achieved last season.
The new coach's abilities are doubted, of course this is quite normal, especially since he has to take over a team when the season has already played quite a lot of matches and the team needs improvement, even though Napoli is not that bad, it's just that the comparison is of course last season so it's not an easy job for Napoli's new coach, and I'm pretty sure the target is no longer to defend the title because it's already very difficult, we could even say the opportunity is closed, Walter Mazzarri just needs to keep his team in the top four until the end of the season to return to playing in the Champions League next season and also continue to perform well in the Champions League this season.
Only teams like Juventus, Inter Milan and AC Milan can do well every season, but teams like Napoli, Lazio or AS Roma always have problems, especially when the previous season has performed well, usually the next one will show a decline.