Sancho has been exiled from training with the squad following his public bust-up with manager Erik ten Hag.
He has also been banished from the first-team dressing rooms and the canteen, forcing the England winger to eat alone at United's Carrington training ground.
Under FIFA regulations, asking a player to train on their own may result in the club being found to have engaged in "abusive conduct," and the Professional Footballers' Association (PFA) are aware there are UK employment law arguments to suggest the treatment is not lawful.
A source told ESPN that the PFA have stressed that United are bound to provide a high level of training and physical fitness for Sancho, but the club believe they have met the standard by assigning dedicated coaches to work with the forward.
The PFA have offered to mediate in an effort to find a resolution in the row.
Sancho has not apologised for publicly calling out Ten Hag in a social media post after he was dropped for a game at Arsenal on Sept. 3 and, as yet, there is no timeframe for his return to the squad.
Ten Hag was asked about the situation following the 3-0 win over Crystal Palace on Wednesday but said he would "not talk about players who are not available."
Ten Hag, according to an ESPN source, has not taken his decision to exile Sancho lightly with the Dutchman considering it a last resort.
A source has told ESPN that he has the full backing of the club, including from chief executive Richard Arnold and football director John Murtough, and that the legal implications have been considered.
Sancho is training on the academy pitches away from the main building at Carrington. He is changing in the same building and, according to a source, he has been told he must lock the door while he does so to meet safeguarding standards because the facility is also used by children.