The Portugal international will go up against the club he left in 2009 in the last 16 of the tournament in February after Real were pitted against United in Thursday's draw.
Ronaldo, who spent six seasons at Old Trafford and still keeps in contact with manager Sir Alex Ferguson, does not plan to celebrate should he find the back of the net against the side he considered "family".
"If I score against United I will not celebrate," he told AS. "I still speak to Ferguson. I miss him a lot, just like I do Manchester United.
"I felt really good there because it was like a family. I had six great years there and still speak to a lot of my former team-mates.
"Thanks to Manchester United I am enjoying my football with Real Madrid. Without them I could not have made it to be the player that I am now."
Real have twice knocked United out of the Champions League and the club's president, Florentino Perez, is relaxed over the draw.
He told Marca: "It's fine, Manchester is fine.
"We've been drawn against them twice and eliminated them twice."