The Scotsman, who signed the Portugal captain from Sporting Lisbon as a teenager in 2003, will come face to face with the 27-year-old, who moved to the Bernabeu from Old Trafford for a record £80m over three years ago, in the Champions League.
"I still have an excellent relationship with [Ferguson] and it will be nice to see him again," BBC Sport quotes Ronaldo as saying. "He taught me the basis of football.
"I arrived in Manchester when I was only 18. He managed my career in the best possible way and for me he is one of the most important people in football."
United will travel to Spain for the first leg of the round-of-16 tie next week.