The Partenopei missed the chance to close the gap on Juventus and Roma at the top of the table after conceding three second-half goals.
"We had to play with sharpness and quality to win. We had three or four promising counters in the first half, then started the second with a mistake. We had a chance, then a mistake, then another chance and another error," he told Sky Sport Italia.
"Of course, it becomes very difficult to get back into a game after making so many mistakes. I don't think these errors have happened before, but they completely changed the game.
"If we don't take our chances and the other team does, then of course we are going to lose. There was also a penalty on Dries Mertens. It is difficult to find space against teams like Chievo, Bologna and Atalanta, but we must learn to score more goals."
Napoli are currently third in the Serie A table.