Jose Mourinho does not know how to craft a team from a collection of very talented, exceptionally well paid individuals. And he undermines the confidence of, and diminishes his players.
No player has left a team managed by Mourinho in recent years better, more confident, more fully rounded, than he was when he arrived.
Alex Ferguson used to demand loyalty from his players, but he gave it to them as well; for example, he would never criticise a player in public (as Mourinho does incessantly) but would give them the proverbial "hair-dryer treatment" in the dressing room in private.
That way, he never lost the dressing room in the way it seems inevitable that Mourinho will do, given time.
Likewise, he would never allow a player to think that he was bigger or of greater importance than the club: Once that began to happen, the player was on his way out. In their day, each of David Beckham, Roy Keane, Ronaldo, Cantona, - among many others - all learned that lesson in turn.
Now, personally, I am not impressed by Pogba's conduct; pride alone should suggest that he play his best when he cost that much and when he commands those silly, stratospheric wages; however, it is telling that Manchester United had sold him ages ago unable to develop his potential, and even more telling and damning that he has always played better elsewhere, fully realising his potential - for example, at this summer's World Cup. In Manchester, he is a shadow of what he could be.
Re West Ham, some of those summer buys were good ones; but transfers - unless it is into a settled and stable team, where development is a priority, can be unsettling and destabilising. It does take time to gel.