The England international has not featured for the Red Devils since the Premier League clash against Aston Villa on October 6 due to a muscular issue, missing the team's last seven matches in all competitions.
However, Mainoo trained on Monday in what was Ruben Amorim's first session since taking charge of the 20-time English champions, and the midfielder could now be available against Ipswich this weekend.
The 19-year-old has made nine appearances for Man United this season, including seven outings in the Premier League, and he is expected to be a vital player under the management of Amorim.
Amorim is unlikely to make a decision on the academy product's involvement this weekend until later in the week, as much will depend on how he progresses through the upcoming sessions.
Mainoo trains under Amorim on Monday
Man United also confirmed that Luke Shaw, Tyrell Malacia and Leny Yoro were part of the session, with the trio continuing to work their way back from injury problems.
Shaw, Malacia and Yoro are yet to play a minute of first-team football this season, but all three are also said to be under consideration to be involved in the squad against Ipswich in a further injury boost for the new head coach.
Man United have suffered a double injury blow, though, with Lisandro Martinez and Victor Lindelof both needing to be assessed.
Martinez was forced to pull out of the Argentina squad due to an issue related to his back and hip, while Lindelof suffered a groin problem while representing Sweden, and it does appear that the latter will be facing a period on the sidelines.
Which other players trained in Amorim's first session?
Amad Diallo was also present on Monday, with the attacker not representing Ivory Coast during the international break due to an issue which he suffered against Leicester City last time out.
Mason Mount, Tom Heaton, Jonny Evans, Casemiro, Marcus Rashford and Antony were also pictured taking part in the session.
Amorim will only have a maximum of two training sessions with his full squad before the Ipswich match due to the amount of players still involved with their countries.
Alejandro Garnacho (Argentina) and Manuel Ugarte (Uruguay) are set to be the final two players to return, with the pair unlikely to be back at the club's Carrington training complex until Thursday.