The Portuguese fired in Real Madrid's equaliser after Danny Welbeck had put the visitors ahead in the first leg of their Champions League last-16 tie.
But after heading home to level the scores, Ronaldo made a gesture to the United dugout.
"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.
"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."
Real's movement posed real problems for the English Premier League leaders, with Ronaldo and Mesut Ozil causing problems for Rafael at the back.
But with their first meaningful attack, on the 20 minute mark United scored. Welbeck claimed his second goal for the season after shrugging off Sergio Ramos to head home Wayne Rooney's corner.
Just ten minutes later, however, Real were level, as Ronaldo added to his tally of 182 goals in 179 games.
Angel Di Maria floated in a cross which the former United winger leapt high above Patrice Evra to head down past David De Gea.
United pressed to take the lead again, with Welbeck coming close towards the end of the first half, but Real carried more threat.
Ozil forced De Gea into a near-post save after the Manchester United back line was caught out by Xabi Alonso's long ball.
Van Persie had a chance to clinch the game for United with 15 minutes to go, clean through in the box on the end of a Michael Carrick header, but mis-hit his shot, giving Alonso time to clear.
Afterwards Sir Alex Ferguson revealed Ronaldo had joined his former colleagues in the United dressing room.
"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."
Ferguson said he would have accepted the result before kick-off.
However, it required a series of outstanding saves from David de Gea to ensure the tie remained deadlocked.
"De Gea was excellent," Ferguson said. "He made three or four top saves.
"He has been improving all season.
"There was one save in the first half which he touched onto the post (from Coentrao) which was superb.
"I am pleased for him. He is an Atletico boy playing here in Madrid, but he has done well."
Jose Mourinho paid tribute to United's performance at the Bernabeu after the game, admitting his side could easily have lost.
"It was possible for them to win 3-1 because they had two big chances when Alonso cleared off the line and Diego made a save," said Mourinho.
"I won't criticise what they did. United did their job well and got the result they wanted.
"They came here not to concede space and goals and they didn't play to win.
"But they know Real Madrid can do it there."
It is widely assumed Mourinho will find himself out of a job should the Spanish giants fail in their quest.
However, the former Chelsea coach insists he is not under pressure.
"I don't feel any pressure," he said.
"I work hard, am very serious and am happy with what I do.
"I have peace of mind and my head is high, but there is no pressure really.
"I have no idea and I am not worried about it."
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