The two teams face each other in the last 16 of the Champions League, the first leg at the Bernabeu next week.
The Portuguese star left Old Trafford for the Spanish capital in a world record transfer fee of £80m in 2009 and next Wednesday's game will be Ronaldo's first against his former team.
Looking forward to the mouth-watering clash, Ronaldo - speaking in an interview given to Champions League magazine - said: "I believe things will go well.
"In my opinion Real Madrid have a better team but we have to show that on the pitch because United are a very united team and always have been in recent years.
"They're a team that always do well, every year they're at the top and therefore a team we respect a lot.
"It will be an extremely difficult match, especially in Manchester.
"I believed we would draw Manchester United. I've wanted to face them for some time and I have great memories from my time there.
"It will be a match in which I will feel a lot. I was there for six years. I have a lot of friends there, people I will see with great pleasure. It will be a match with a lot of emotions."
As for the United manager, he added: "I still have an excellent relationship with him and it will be nice to see him again.
"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."