Walter Mazzarri believes Napoli's participation in the Champions League this season could hurt their chances in the Italian Serie A.
Napoli entertain Villarreal on Tuesday at the San Paolo Stadium looking to kick on from their positive start in the competition.
Two weeks ago they came back from England with a valuable point following a 1-1 draw with Manchester City.
But the twin objectives of league and Europe could prove too much for Napoli, according to Mazzarri.
The coach was widely criticised for resting six players in their midweek 1-0 defeat at Chievo and despite restoring the majority of his first choice line-up against Fiorentina on Saturday, they still fired a blank in a goalless draw.
Mazzarri believes he was right to switch his team around and says if he didn't they wouldn't succeed either at home or abroad.
"Fiorentina have excellent players, they're a good team and on top of that they're not involved in the Champions League, unlike us, and so they were fresher than us," he said.
"Being in the Champions League can cost you 10 points in the league and I knew that.
"And then you can't forget that we've got Villarreal and then Inter (Milan next weekend) coming up. I'm sticking to my path, I had to make changes in Verona against Chievo to protect my players.
"(Christian) Maggio was the only player who played in every game and he got injured (against Chievo).
"That would have happened to the others if I hadn't rested them. Things went badly in these two games and we lost on Wednesday and drew (on Saturday)."
Napoli had started the season well with victories against Cesena and AC Milan, by the same 3-1 scoreline, as well as the draw with City.
But Mazzarri insists people shouldn't get carried away by those results.
"I never kid myself, maybe those two victories made people think the league was over but that's not the case," he added.
Apart from Maggio, Napoli should be at full strength for the match while Villarreal have included Italy forward Giuseppe Rossi in their squad despite him limping off against Athletic Bilbao on Saturday with a knee problem.
Rossi was linked with a move to Napoli in the summer but nothing materialised as any possible bid would have depended on Napoli selling Ezequiel Lavezzi.
The two sides clashed in the Europa League knock-out stages last season with Villarreal progressing 2-1 on aggregate despite Napoli having the better of both games.
However, Villarreal are desperate for a result having started their Group A campaign with a home defeat to Bayern Munich.
They have also had a poor start to their league campaign, winning only one of their first five games and even suffering a 5-0 humiliation at the hands of Barcelona.