Earlier this month, the 19-year-old missed the 3-3 Europa League away draw with Porto due to a small muscle issue.
Mainoo returned to action to play 85 minutes against Aston Villa in Man United's final match before the October international break.
The youngster was forced to pull out of the England squad, before Man United confirmed that Mainoo had sustained a muscular problem in the goalless draw with Aston Villa.
As a result of the injury, Mainoo has been forced to miss Man United's last three games against Brentford, Fenerbahce and West Ham United.
Mainoo provides promising injury update
Prior to Sunday's defeat to West Ham, Mainoo took to Instagram to offer an update on his recovery, hinting that he is making progress in his bid to return to full fitness.
The England international shared a series of pictures, including ones of him working on an exercise bike and sitting on a training pitch.
Along with the images, Mainoo simply wrote: "Be right back."
The update will offer hope to Man United supporters, but it is still unclear whether he will return before the November international break.
Man United will compete in four consecutive home games before the next international window, beginning with Wednesday's EFL Cup tie against Leicester City.
The Red Devils will then face Chelsea and PAOK in the league and Europa League respectively, before they face Leicester again in a Premier League clash on November 10.
Mainoo one of several injury absentees
Unfortunately for Manchester United boss Erik ten Hag, he is having to cope without several injury absentees in addition to Mainoo.
At the moment, Ten Hag is unable to call upon Leny Yoro, Harry Maguire, Luke Shaw, Tyrell Malacia and Mason Mount.
Antony recently joined the absentee list after he was forced off with an injury in Thursday's Europa League draw with Fenerbahce.
As well as coping with an injury crisis, Ten Hag is also having to deal with the mounting scrutiny on his position as Man United manager.
Having collected just 11 points from nine matches, Man United are currently sitting in 14th position in the Premier League table.