Is that true? He seemed like he was a pretty popular member of the squad and someone that they valued quite highly - the most controversial thing he did at Dortmund, if I recall, is a public display of support for George Floyd's family... which isn't even that controversial tbh.
I wouldn't be surprised if he got lazy because he's on big wages and that's caused some attitude problems, that happens with some players. I do think it's a bit strange of man management when you've got an expensive high profile player who's said he's been undergoing mental health treatment getting frozen out like this by the manager.
It's hard to come back from being told to train alone. Whenever that's happened with us, the player's been pretty quickly moved on. It usually demonstrates a complete breakdown in the relationship between the player and the manager and the manager no longer wants that player around to "infect" the squad. It takes a lot to repair a breakdown like that. I imagine he's on his way out as soon as the transfer window opens.
Probably bad planning from United's part not finding a club willing to at least take him on loan or convince him to take the untold riches of Saudi Arabia once the window closed.