Today I saw a player who is positioned about 20m too far back - Sanchez was making blocks in our penalty box...
This is really the key issue with Sanchez. He is not a winger. If you think about the Football Manager positions, for those who are familiar with the game, he's an inside forward. He needs to have freedom, and to pop up all over the place along the forward line, not be a right-sided midfielder in a stringent formation.
Mourinho has never trusted wide players or creative players. They always go backwards under him, or he loses trust in them. I remember the first Chelsea team he had, they were absolutely flying with Robben and Duff (and look at the career that Robben has had since). But he ended up marginalising both of them. He also signed Wright-Phillips and never trusted him. Martial and Rashford have both gone backwards under Mourinho, and he famously let both Salah and de Bruyne go (obviously de Bruyne is now a fantastic midfielder, but he was primarily a wide player originally).
If you watch this video, you can see Sanchez's last two goals for Arsenal:
Particularly the second goal, can you imagine him being allowed to do that for United? He's never going to be afforded that sort of freedom, whereby he feels he can run in behind the defence and be the furthest player forward. If you don't give him that then you're totally wasting him.
It is true that Sanchez's last few months at Arsenal weren't as good as his earlier period, but don't forget (a) he wanted to leave at this time, and obviously wasn't motivated, (b) he still scored 8 in 22 appearances, and several of those were as sub, whereas he has scored 3 in 24 for United, and one of those was a rebound from a penalty that he missed. If you look back, he started last season slowly for Arsenal, being rested by Wenger, but by October he had picked up, and scored 7 goals between 22nd October and leaving. If he'd stayed, he probably would have got 20+ goals again.
I can't believe that he's completely shot, nine months after scoring those goals against Palace, which were literally days before he joined United. He's just not being used properly, he's probably disillusioned because he clearly should have joined City, and Mourinho just doesn't trust players like him.
He got by for a long time on playing rigid, structured football that was very defensively solid. But the game has moved on, and he hasn't.