"He impressed me so much I made the decision I don't want this kid to go on loan, I want him to stay, and in our first match of the Premier League season he started against Hull City," Mourinho said.
"In the second match we got to Old Trafford and after this he come to me and said, "I want to play every match, I want to play every minute". He put a lot of pressure on that.
"He wanted to leave, he wanted to go to Germany where he was previously on loan and so happy, and his decision was to go. He put big pressure on that and it worked very well for him.
"The story is there and the quality of the player is absolute, he is top five in the world."
Mourinho added that De Bruyne was not prepared to bide his time and wait for his opportunity to make an impact at Stamford Bridge.
"When he was supposed to leave on loan and I made the decision for him to stay, I saw those qualities. He did not have the experience and maturity he has now, but his feet and his vision was already there," he continued.
"He knew very well what he wanted and he was not ready to have that season at Chelsea where a lot of good players were in the same team, he didn't have the patience.
"He was not ready for that, he was coming from a season in Germany where he was playing every minute of every match.
"Sometimes us coaches make mistakes, but that was not the case. I saw he had the potential to do it, but he just knew perfectly well what he wanted."