The Partenopei were knocked out of the Champions League at the playoff stage and have won just one of their opening four league matches.
"I cancelled the team's day off because when we need to improve on something, we have to use all the time at our disposal to work on it," he told reporters.
"We have to improve in defence and we are working on that. Are there psychological issues within the team? If you don't win, it means something needs to be improved. We are lacking confidence and you can't just buy that at the supermarket.
"The other day I said the team was in good shape and deserved more from the last few games. The results do take something away from your confidence and we'll get that back if we start winning again. The key to emerging from this moment is hard work."
Napoli are currently 10th in the Serie A table.