This decision is made to comply with the Premier League's financial rules, which dictate the amount of money teams can spend on transfers.
The players who are not for sale are Bruno Fernandes, Amad Diallo, Patrick Dorgu, and Harry Maguire.
However, several other players, including Kobbie Mainoo, Luke Shaw, Manuel Ugarte, Casemiro, Rasmus Hojlund, Matthijs de Ligt, and Joshua Zirkzee, may be sold by the club.
Alejandro Garnacho, a winger from Argentina, is also available for transfer, with Chelsea and Atletico Madrid reportedly interested in signing him. United are willing to sell him for £65m.