This is nonsense. The club and manager don't get to decide whether someone is suffering from depression/mental issues or not, and in general mental health has never been a high priority for clubs in the past. They just treat them as financial assets that either deliver or don't, and if they don't deliver, they're not worth anything.
That's not entirely true, clubs build an entire support system around players, have medical doctors, psychologists etc on staff. But ultimately, if a player can't handle the demands, or is too mentally or physically fragile, it doesn't really matter what the reason is, the club is justified in moving them on if they can't cut it. Like I've said before, these are elite performers at world class organisations, more is demanded from these people than from the likes of you and me, and they get paid handsomely for it. Not everyone is strong enough for it and sadly it looks like Sancho is one of those. He can always go play football somewhere less demanding, nobody is stopping him.
Also, even if United was rubbish at recognising these supposed mental health issues, he's on £16 million a year, he can afford to get a referral to the world's greatest doctors if he thinks he needs more help. He's a fully grown, adult multi millionaire FFS. He lives in very different world to the rest of us, with access to unimaginable resources that few others get. Don't make excuses for him, make him accountable.