Sometimes not all managers and teams click, that doesn't mean that the team was terrible, but it also doesn't mean that the manager was bad. That is a risk that the managers are taking, Garcia wasn't the manager to lead Napoli, that doesn't mean Garcia is a bad manager, the downside is that he will be labeled as the guy who took over a champions Napoli team and "ruined" them, not like they were doing that terrible to be fair, they are doing fine.
I think that's going to be the label for him and he will have harder time finding a job, or at least at a high level club. This is going to be fine though, because if he finds a club he clicks well with, then he is going to do better and he will show himself to be a good manager again.
Well definitely a coach working in unified understanding with the players in very important for a team because if a team doesn't align with the coach then surely the club will begin to have problems. So for the growth of a club to occur, the players and the coach needs to be in unism. I think Garcia doesn't understand Napoli's pattern of play the way Spalletti did and besides he doesn't also have good understanding with the players so in this case the best Napoli could do is to employ the service of another coach that will understand Napoli pattern of play more better.