After not earning a permanent move to Chelsea, Sancho was offered for sale by United at a starting price of £ 25 million.
Juve asked after Sancho early last week before Napoli made contact on Friday.
Sky Italia says that both Italian giants shop for a new winger this summer and are willing to take a gamble on Sancho.
Sancho, who has only passed on his contract for a year, has to take a considerable wage reduction to complete a move to Italy.
It has been suggested that he has already agreed personal conditions with Napoli, although a fee with United is difficult to establish.