Ronaldo scored Real Madrid's first-half equaliser after Danny Welbeck had put the visitors ahead in the first leg of the Champions League last-16 tie at the Bernabeu. Instead of celebrating the former world player of the year opted to acknowledge the Real fans before making an almost apologetic gesture to the Red Devils dug-out.
"I was in Manchester for six years so tonight was very emotional for me," the Portuguese said. "I scored a goal but I didn't celebrate out of respect."
He added: "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."
Sir Alex Ferguson revealed Ronaldo went into the United dressing room to speak with his old team-mates.
"I spoke to him. He is sitting in our dressing room, talking to the lads," said the United boss. "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 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 Messi."