McTominay has been heavily linked with a move away this summer, having entered the final year of his Man United contract.
While the club have the option to extend his deal by a further year, they are willing to bid farewell before Friday's transfer deadline.
Fulham were among McTominay's suitors before they failed with a couple of bids and subsequently turned their focus to signing Sander Berge from Burnley.
Serie A club Napoli are also among McTominay's admirers and they have remained in negotiations despite failing with a €25m (£21.2m) bid.
Napolo reach McTominay agreement
Napoli's persistence has ultimately paid off with The Athletic reporting that the Italian side have reached an agreement with Man United to sign McTominay.
The report claims that Man United have agreed to sell McTominay for €30m £25.4m) and will receive a percentage of any future sale.
The transfer would help Manchester United avoid any issues with the Premier League's financial rules because selling the academy product would be registered as a pure profit.
There is a suggestion that McTominay still needs to give the green light before the two clubs can finalise the transfer.
Napoli sporting director Giovanni Manna has been in England to negotiate the move as well as two other deals for the Italian club.
Manna is looking to finalise moves for Chelsea striker Romelu Lukaku and Brighton & Hove Albion midfielder Billy Gilmour.
McTominay heading towards Man United exit
McTominay is on the brink of ending his 22-year association with Man United, having made his way to the first team after progressing through the club's youth system.
The 27-year-old has featured in 255 competitive matches for the Red Devils, chipping in with 29 goals and eight assists.
McTominay made what could be his final appearance in Saturday's clash with Brighton & Hove Albion, coming on as a 79th-minute substitute in the 2-1 defeat.
If he does complete the move, McTominay will add to Antonio Conte's current midfield options which include Stanislav Lobotka, Frank Anguissa, Gennaro Iaccarino and Coli Saco.