With the recent changes at Napoli, it'll take a miracle for the them replicate their great performance of last season. The new manager and the new players would certainly have to work together for awhile to better understand themselves and find the best style of play that'll work for the team.
Defending a league title is considered to be harder than winning because other clubs in the league will be very much wary of the defending club.
Sometimes with a new coach, you don't necessarily have the opportunity to work as hard as last season. Napoli were good at the end of last season because they managed to dominate the Serie A with a wide margin of victory points; no other club could be like that, meaning that Luciano Spalletti has made the best contribution to Napoli and they should have kept him because Rudi Garcia may not necessarily be able to give them victory like last season.