The winger is believed to have signed a pre-contract with the Partenopei, and was set to bring his move forward to January. However, Younes turned down the deal last month.
"According to the information I have, Younes has already signed for Napoli," Overmars told De Telegraaf.
“As a result there's no way he can stay with us for next season."
There's suggestions Napoli intend to immediately sell Younes when he arrives this summer.