The brilliant Portuguese forward established himself as one of the finest players in the world during a six-year stint at Old Trafford before moving to Spain in 2009.
And, lining up against his old club in the first leg of a Champions League last-16 tie at the Bernabeu, Ronaldo got the equaliser in a 1-1 draw, rising majestically above Patrice Evra after half an hour to head home an Angel Di Maria cross.
But the 28-year-old did not celebrate wildly, showing respect for his former club by simply acknowledging the jubilant Real fans.
"I was in Manchester for six years, so tonight was very emotional for me," said Ronaldo. "I scored a goal but I didn't celebrate out of respect.
"Maybe United are favourites now, maybe not, anything can happen. Sometimes we are better away than at home, so we will see.
"It will be amazing to play at Old Trafford again after four years. It will be another emotional night."
In his post-match interview, Manchester United boss Sir Alex Ferguson said of Ronaldo: "I spoke to him and he is sitting in our dressing room now, talking to the lads.
"He had six years with us and I am proud of how he flourished.
"I blamed Patrice Evra for not challenging for his goal until I saw the replay and felt a bit stupid.
"His (Ronaldo's) knee is about as high as Evra's head - phenomenal. I don't think any other player in the world can do that, certainly not (Barcelona's Lionel) Messi."