The Real midfielder is now on-loan with Borussia Dortmund.
"It was a great opportunity to play with him [Ronaldo]," he told AS. "He is an incomparable professional, he's incredible. In one year, I learned a lot from Cristiano. Maybe some people will not think so, but he is very humble and friendly.
"Moving to Madrid was a dream. I did it. I had an injury in the first training session, and if that had not happened, I'm sure it would have been different for me.
"But that is football. I don't want to return to the past. I'm at one of the best teams in the world, I never let my head drop and I think I will be very successful."