Napoli will not only participate in Seria A but they will also play in the Champions League. It's going to be a tight schedule for them and I don't think that the players are used to it. They were successful last season because they were more focused on only Seria A, while Inter mindset was divided into two competitions and Inzaghi took UCL as his priority.
Fatigue and injuries can be a big challenge to Napoli next season.
That's not an excuse though, since Inter were not the only Italian team in the Champions League and also participating in other competitions, infact all big clubs in the Serie A suffered injury woes and fatigue too so that doesn't stop Inter from winning the league, if a teams depth is very okay then there won't be any reason to worry more about injures or fatigue, remember that Napoli's coach is a very good tactician and he won't focus on the Champions League for now since the squad is not very balanced for that competition yet, he'll rather try to win the Scudetto consecutively, I think so.