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.
The changes that have occurred in the Napoli squad have made their chances of retaining the title even smaller, especially since the coach who helped Napoli win the Scudetto last season has left the club. Napoli status as reigning champions will certainly be a burden on the new manager, Rudi Garcia is required to keep Napoli on track as Spalletti has done last season.
Serie A has always been won by three different clubs in the last three years, Inter Milan, Milan and Napoli. This season there will be more fierce competition and other clubs also want to win the Scudetto, Juventus will try to return to their best form to win the title after three years of failing to finish in first place. It is difficult for Napoli to defend the title after other clubs are very active in the transfer market to bring in players who can add to the strength of the team.