I would add a couple of things: I don't know why De Laurentiis, the Napoli president, chose to hire an ex manager like Rudi Garcia after winning the Scudetto, that guy is not a coach, he's a disaster, everybody knew Napoli's season was going to be a failure.
Garcia wasn't that bad, at least Napoli was still in a CL qualifying spot when he left, it was the next two changes that really doomed them, if we multiply his results, they would have still managed to cling to 6th, still bad but not as bad as it turned out to be.
Mazzarri, that was a disaster, and I see no reason why you would put a manager who has always been sacked for poor performance in charge of a team that still has some goals in mind.