Patrick De Koster took a swipe at Mourinho for letting De Bruyne leave Chelsea during his time in charge at Stamford Bridge.
The agent said: "To this day Mourinho struggles to admit he has made a mistake. He said Kevin was not yet ready for the first team.
"He had four options in 2015. He could have stayed at Wolfsburg, but by then he had seen and done it all.
"He could have joined Bayern Munich, but they never go over a certain limit with paying their players.
"Paris Saint-German were a third option, but there are too few top-level matches in France.
"Fourthly, Manchester City, who play a game of Champions League standard almost every week.
"We made a good choice in joining them. Since then he has become 'King Kev.'"