This result ends Napoli's 100 per cent record and raises doubts over their title credentials.
Mazzarri said: "I never said we'd be fighting for the Scudetto, as I only ever talked about the growth of a squad and everyone knowing what to do on the field. Clearly teams who have been fighting for the top of the table for a long time have a different mentality in these situations.
"Perhaps having an extra man so early made us think we'd already won it and that proved counter-productive. Some players seemed tired and off form, but I allowed them to rest, so there's no excuse."