An interview published in Belgian newspaper Het Nieuwsblad suggests the 26-year-old will pick the Italian runners-up ahead of a London club and possible Barclays Premier League football.
Herman Mertens told nieuwsblad.be: "Dries sent me a text message saying he expects to go to Naples next week.
"There doesn't seem to be anything standing in the way of the move. Dries really wants it.
"His girlfriend let slip that they would have preferred to go to London but when a club like Napoli comes knocking, you should not have any doubts. It is a unique opportunity."
Belgium international Mertens scored 16 goals in 29 Eredivisie appearances for PSV as Philip Cocu's men finished behind fierce rivals Ajax in the 2012-13 season.
A move to Napoli would see Mertens work with former Liverpool and Chelsea manager Rafael Benitez, who will be unveiled as the Italians' new boss on Friday having signed a two-year contract at the end of May.