#1 Fallout with the players
The primary role of a manager is to keep the dressing room hungry and motivated. Jose Mourinho is a serial winner who won major trophies wherever he has managed.
His appointment at United was to ensure that the Red Devils could win the Premier League again for the first time since Fergie's retirement. However, one thing that the board might not have thought about is Mourinho's third season syndrome.
In his third season, Jose generally falls out with his players. Previously in case of Real Madrid and Chelsea, he lost the dressing room which led to his dismissal. At United, the Portuguese fell out with one of the most expensive players in the world, Paul Pogba. Footages circulated of a suggested spat between the manager and the world cup winner on the training ground.
Apart from that, he also criticized Luke Shaw and Antony Martial for their performances. The England International was almost cast out by Mourinho, but he fought his way into the good books of the manager.
Other players like Bailly and Valencia were reportedly unhappy with his philosophy. All these culminated into a miserable dressing room which eventually led to poor performances.