Hamsik moved to Napoli in 2007 for €5.5. million.
"I regret letting Hamsik go to Napoli for such a low price," Corioni told Radio CRC on Wednesday afternoon.
Hamsik was rumoured to be a 30m target for Milan this summer, but the attacking midfielder will remain at the San Paolo.
"Hamsik is not a top champion, but he is still irreplaceable. Players with his characteristics should not be sold."