Ronaldo will come up against his former club for the first time next week when United visit the Bernabeu in the first leg.
Jose Mourinho's side have struggled for consistency in La Liga this season but Ronaldo thinks that the difficult draw can help bring out the best from his team.
He told the official Champions League magazine: "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.
"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.
"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."
Mourinho's men are currently languishing 16 points behind Barcelona in La Liga and sit seven adrift of city rivals Atletico Madrid.