The midfielder does not feel he fits into the vision of head coach Ruben Amorim, who has yet to select him to start a Premier League match this season.
We revealed last week how Chelsea are keeping an eye on his situation, and we understand there are others too, with Napoli leading the way.
A transfer could still become a possibility, we understand, as United are frustrated by the fact that he is showing a lack of patience.
Earlier in the campaign, it was expected within Old Trafford that Mainoo would sign a new contract. However, talks have not been progressing, and the player believes he has no defined role while Amorim remains in charge.
That view is not comprehensively shared by the club.
Mainoo has requested permission to leave during the January transfer window, but he was informed last week that United have no intention of allowing a departure at this stage.
We understand the club feels the 20-year-old England international should remain patient and consider the bigger picture.
With uncertainty surrounding Bruno Fernandes’ long-term future, as well as questions over Casemiro and Manuel Ugarte, United believe opportunities will open up for Mainoo to establish himself in the side again after this season.
From the player’s perspective, that pathway has been difficult to see - and perhaps difficult to believe. That is reflected somewhat in the fact his half-brother recently wore a T-shirt declaring the words: ‘Free Kobbie Mainoo’.
There is a growing perception that Amorim has issues with Mainoo and that the midfielder does not fit the team’s tactical structure. United sources insist this is not the case, but tensions remain as January approaches.
United are torn over Mainoo now - in the sense that his attitude towards the situation is not as they would like it to be. It means there has been some consideration about letting him go.
Mainoo missed the weekend’s match against Aston Villa, reportedly due to a minor injury. If he is to earn another opportunity at Old Trafford, this is a crucial moment.
With Fernandes currently injured - and Amorim previously suggesting that he is Mainoo’s direct competition - this is the ideal chance for Mainoo to get a start.
Manchester United face Newcastle United on Boxing Day, and it remains to be seen whether Mainoo will be available for selection.
Ultimately, a moment of mutual understanding and respect will be required if this situation is to be resolved.
Follow United's Boxing Day fixture against Newcastle with Flashscore.
