The Portugal captain came up against his old employers for the first time during their last-16 first-leg tie on Wednesday, and cancelled out Danny Welbeck's opener to earn the La Liga champions a 1-1 draw at the Santiago Bernabeu.
The 28-year-old left United in 2009 after a six-year stint at Old Trafford, and he admits that he is looking forward to returning to his old stomping ground.
"I was in Manchester for six years, so tonight was very emotional for me," Ronaldo told Sky Sports News. "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."
The return leg takes place on March 5.