Spalletti's departure will certainly have a huge impact on them for next season and the exit of several key players will greatly affect the team's performance. Napoli showed quite a good performance last season so that everyone hopes that the new Napoli coach can continue this performance, thus creating high expectations and in my opinion it is very unlikely in his first season and also possible because these high expectations burden this team, especially the coach new Napoli Rudi Garcia.
There is not several key players who left as you said, only Kim Min-jae with a contribution above the average of other players who officially left Napoli this season. In this case, the key players in important positions in Napoli have so far perform well, it's just that we don't know how the new Napoli coach uses every player in the team.