According to transfer expert Ben Jacobs, the 27-year-old Scotland international is on his way to southern Italy to undergo his medical and sign with Napoli.
United will receive €25m from Napoli for McTominay.
His contract expires next summer, with the proviso that United have the option of a 12-month extension.
However, should his medical be successful, McTominay will cut all ties with his hometown club.