The Partenopei's title hopes were all but ended, with the loss leaving them 15 points behind league leaders Juventus.
"We did not expect this defeat," he told Rai Sport. "We made too many mistakes, many of which were my own.
"I feel responsible, but now we must look ahead to midweek."
Napoli visit Roma in the first leg of their Coppa Italia semi-final on Wednesday.