The pair spent three years together at Santiago Bernabeu, with Ronaldo scoring an astonishing 168 goals in 164 games in that time.
And although they reportedly didn't always see eye to eye at los Blancos, the current Chelsea boss was full of praise for the 28-year-old forward.
"Coaching him was the highlight of my career," Mourinho told France Football.
"He's the most professional player I've ever met.
"A coach and player may have their differences at a given time, but it ends there. I don't have any problem.
"Being at a club with Cristiano Ronaldo is the best thing to have happened to me.
"Why? Because if I let him pass ahead of me he would be mobbed immediately, and I would be free to pass by in no hurry.”
Ronaldo has already scored 24 goals in 17 games this term in all competitions and is one of the leading contenders for the Ballon d'Or.