Manchester United's Mainoo out for a few weeks due to muscle injury
Mainoo missed England's Nations League games against Greece and Finland due to the injury along with teammate Harry Maguire, who is also out for a few weeks after picking up an injury during the 0-0 draw at Villa.
Forwards Alejandro Garnacho and Amad Diallo, who also missed international duty this month due to injury and illness, are back in training ahead of Saturday's home game against Brentford, the club said.
Morocco defender Noussair Mazraoui had also returned to training after being substituted at Villa due to illness.
Injuries have also sidelined Luke Shaw, Tyrell Malacia, Mason Mount and Leny Yoro, adding to United's struggles as they sit 14th in the league with eight points from seven matches.