Ronaldo faces his old club in the Champions League next week.
He told Champions magazine: "I have wanted to face United for some time because I have great memories of my time there.
"Sir Alex 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.
"I still have an excellent relationship with him and it will be nice to see him again.
"It will be a match in which I will feel a lot. I was there for six years and I have a lot of friends. It will be a match with a lot of emotion."