The Partenopei were on the back foot for most of the match and were ultimately punished by goals from Leonardo Bonucci, Arturo Vidal and Fabio Quagliarella, all in the second half.
"You can lose to Juventus in Turin, the only question is how this will happen," Mazzarri told Sky Sport Italia.
"We played on par with Juve until they scored their second goal, then we stopped working as a unit and we became nervous.
"After so many matches, we have lost a bit of sharpness, while Juventus are defensively very solid and we suffered a lot against their physicality.
"Giorgio Chiellini can mark all three men by himself and it is impossible to get past him. We were unlucky in Juventus' first goal, but we reacted well and went in search of an equaliser.
"But after Arturo Vidal's wondergoal, we dropped our level considerably and we were never able to get back into the game."
Napoli lie in fourth position in the Serie A standings, three points behind Lazio, who lost to Parma 3-1 on Saturday.