The 23-year-old was sold to Wolfsburg for £18 million after making just nine appearances for Chelsea but the reason he didn't feature more often prior to his transfer never surfaced.
A young footballer like De Bruyne needs regular playing time and under Mourinho that was not happening. His first season in the Bundesliga was a success, notching 15 goals and 25 assists in all competitions and winning the DFB Pokal.
The one part of his game that may have turned Mourinho off is his lack of defensive contribution. This is a fault the manager allegedly picked out with Juan Mata, a Chelsea fan favourite at the time, but at Manchester United the Spaniard has ever so slightly improved that aspect of his game.
“I still don’t understand why Jose Mourinho let me leave," De Bruyne told reporters, as per Goal.
"I never discussed it with him. I started to play and then it was finished.
“After four months, I asked him to leave. I didn’t want to stay at a club where I wasn’t even playing for 10 minutes. He said it was my attitude in training? It is an excuse he can use.
“But no-one can say that as they haven’t seen me train. I don’t think that in Belgium or in the Bundesliga they have complained about this. That is the way it is. I am not angry. When I was transferred to Wolfsburg, Chelsea got their money. Everyone was happy.”
“I have not caused any problems. I am going to take my time," he added.
“But the squad really has to be reinforced because it will be harder next season to play both the Bundesliga and the Champions League.
“If I leave, I have no preference where I go. In my head, it is 95 % I won’t go to Russia or Italy. It is not my style of play.
"But England, Spain, France or the Bundesliga, yes. They are the best leagues. There are some very good teams. That appeals to me.”