The 27-year-old has spent his entire club career to date at Old Trafford, having first become involved with the United youth system aged just five.
He is therefore part of 'homegrown' quota in United's current Premier League squad.
However, according to Corriere dello Sport, McTominay is now poised to leave United for Napoli as the two clubs are very close to finalising an agreement.
It is claimed that Napoli will pay a transfer fee in the region of €30million (£25.4m) for McTominay, who has featured as a substitute in all three of United's games so far this season.
Midfielder McTominay, 27, has played more than 250 United games since his debut in 2017
His latest appearance came on Saturday in a 2-1 defeat at Brighton. Fellow Scotland midfielder Billy Gilmour also played in that.
Gilmour has also been linked with interest from Napoli this summer but it is said that McTominay has since become the Italian club's priority midfield target.
Napoli finished 10th in Serie A last season, having won the title in the previous campaign.
They have since signed Leonardo Spinazzola, Rafa Marin, Alessandro Buongiorno and David Neres.
Chelsea striker Romelu Lukaku is also close to sealing a move to Naples.
Belgium striker Romelu Lukaku, 31, is also close to completing a move to Napoli this summer
Selling McTominay would see United free up space and funds to support their pursuit of Paris Saint-Germain midfielder Manuel Ugarte.
Ugarte has not featured in either of PSG's first two games of the new Ligue 1 season, having made it clear that he wants to join United.