The Portugal winger became the most expensive player in football history when he left Old Trafford for the Santiago Bernabeu for £80 million in 2009.
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"I believe things will go well," he told Champions magazine. "In my opinion Real Madrid have a better team, although we have to show that on the pitch because United are a very united team and always have been in recent years."
Ronaldo admitted that he expected to find his return to Manchester emotional after his hugely-successful six-year spell with the 19-time league champions.
The 28-year-old claims he maintains an "excellent relationship" with Sir Alex Ferguson, who spent over £12 million to sign him from Sporting Lisbon in 2003.
"I have wanted to face United for some time because I have great memories of my time there," he added.
"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."