Ronaldo will face former United team-mates for the first time since moving to Spain in 2009, when he was the subject of a world record £80 million bid.
Real Madrid currently sit in third position in La Liga, 16 points behind leaders Barcelona, while United are nine points clear at the top of the Premier League. However, the Portugal international insists his side have a greater pool of talent to draw from in the two-legged tie.
"I believe things will go well. In my opinion Read 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," he told the official UEFA Champions League magazine.
"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."
Ronaldo, who spent six years at Old Trafford, admitted it would be an emotional occasion for him when he takes the pitch at the Santiago Bernabeu next week.
"I actually said beforehand that I believed we would draw Manchester United," he said. "I've wanted to face them for some time. 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."
Claiming he will not celebrate should he score against United, Ronaldo says he still feels admiration for former boss Sir Alex Ferguson.
"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," he added.
"I still have an excellent relationship with him and it will be nice to see him again," he explained.