The two European heavyweights will collide in the UEFA Champions League round of 16 on Wednesday night when the Premier League side visit the Bernabeu for the first leg.
Ronaldo, who spent six seasons with the Red Devils between 2003-09, has nothing but fond memories of his time at Old Trafford.
"Yes Manchester was a very good cycle for me," he told The Sun. "I won everything there. It was a fantastic part of my life there.
"And maybe it was the time I enjoyed my football the most because I was so young.
"To be at Manchester United - one of the biggest clubs in the world - at just 18! It was a dream!
"And to play alongside those players you had only seen before on television - maybe that's why I feel I enjoyed it there the most."
Ronaldo is looking forward to a reunion with former boss Sir Alex Ferguson on Wednesday night, who he regarded as a father figure during his time in Manchester.
The Portugal international, who hit a hat-trick for Real in the 4-1 win over Sevilla on Saturday to fire an ominous warning to his old club, added: "I arrived at United aged just 18 years old and he taught me how to grow up properly in the football world.
"Who better? He has so much experience - 1,000 players have passed through his hands. So he knows and appreciates what young players need.
"And what he taught me was the importance of decision making. How important it is to make the right decisions on the pitch and off it.
"That's why I say he was like a father to me. He taught me everything, he helped me so much, guided me on so many important things.
"That was in football and in life because his influence and interest doesn't stop at the touchlines. He cares about you as an individual.
"He is a very human person who understands what you want and what you need.
"So I owe him a lot. That's why I keep in touch and that's why it will be great to see him."