Reja was dismissed last season and since has taken over the reigns at Croatia's Hajduk Split.
"I'm thrilled for Napoli, the team is playing very well and I think they might have had even a few more points," said Reja.
"If they can continue like this, the road to Europe will be clearing. The experience of last year, however, must not be forgotten.
"The improvements of German Denis? First, the guy has a good head on his shoulders and wants to do well. I told him to lose some weight while we worked together."