The 18-year-old is one of the most exciting young talents at the club and is firmly on the radar of Erik ten Hag and his coaching staff after an impressive pre-season campaign.
Mainoo has made the bench for each of United&aposs last two Premier League matches, including the weekend win over Fulham, but has yet to make a senior appearance so far this season.
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United fans had hoped he would change that when they face Copenhagen in the Champions League on Wednesday night, though they will have to wait again.
Mainoo was not part of the senior United squad that trained ahead of the match on Tuesday morning, with his focus on the Youth League fixture against the same opponents instead.
The Manchester Evening News understands Mainoo was instead involved in training with the U19s and will play for them on Wednesday afternoon as he is eased back into action following his ankle injury.