Jose Mourinho's biographer Manuel Pereira says that the manager's issues with Paul Pogba came down to the Portuguese's desire to control his players.
The manger had highly-publicised issues with the French midfielder, who has been unleashed under new manager Ole Gunnar Solskjaer.
A known extrovert who enjoys social media, Pogba never truly flourished under Mourinho before the manager departed the club midway through the season.
Pereira, who also said Mourinho was too demanding for some players, added the strain was caused by Mourinho's desire to control his players, a tendency that doesn't work as well in modern football.
"He is certainly not someone who controls the privacy of his players. That is not his right. When it comes to football, he likes to have full control," Pereira told Goal and Spox.
"Mourinho quickly becomes suspicious of players who do not reach their maximum in training and games. This can happen once, twice or three times, but not regularly. He does not want them fooling around, but disciplined and playing football. That's what they're paid for.
"If a player gives 100 per cent, Mourinho has no problem with him. Then there is nothing to control. But when Mourinho feels it is not the case and the player feels he has to do less, it becomes difficult.
"Most clubs today believe in the professionalism of their players, that they pay attention to themselves and their diet and in the middle of the season do not drink alcohol or go to parties.
"However, the example of Ousmane Dembele shows well that there are still a few players who struggle with it. At Real Madrid, for example, every player had to be available for a caregiver at midnight in the past. Such rules are inconceivable today. It is even more important for a coach to have someone to represent him on the pitch."
Pereira went on to say that a lack of leadership at United was another issue that plagued the relationship.
Mourinho had a number of respected veterans at each of his prior clubs, but Pereira says that was not the case during his time with the Red Devils.
"Exactly. A respect person who communicates the ideas of the trainer to the rest of the team. You could say: A coach among the players," he said.
"Mourinho always had that before his time in Manchester. For example, Xabi Alonso at Real Madrid, Frank Lampard at Chelsea or Javier Zanetti at Inter Milan. It is always difficult to lead and coordinate a group of 25 men, especially if you are a demanding person like Mourinho. Only a few coaches can do that.
"I remember when Diego Simeone almost always set up Tiago in his early years at Atletico Madrid. Tiago was not a footballer from another planet, but an incredibly strong player, a good communicator who internalised and passed on Simeone's ideas and ways of thinking. Later, Gabi took over this role, today it's Diego Godin.
"At Manchester I don't see such a leader anymore. There is no Paul Scholes, Ryan Giggs or Rio Ferdinand anymore. With such leaders in the team Mourinho would have had it easier."
Pereira also said that Santiago Solari will be feeling pressure at Real Madrid with Mourinho now available while adding that he would expect the manager to jump to a new role in the summer.