Napoli coach Walter Mazzarri had no excuses for the setback.
"It was a bad day and too many players didn't perform the way they usually do," he said. "We knew that Chievo would make this a very athletic and intense encounter, putting pressure on us to then go on the counter.
"A defeat can happen and clearly we have to learn from this match for our future growth. Perhaps I should've made more changes and rotated the squad the way I did against Lecce.
"At times playing a midweek round can cause confusion and disrupt our usual week of preparation, so it's not always easy to make the right decisions."