Ronaldo slams fairweather Real Madrid fans
"We know that we have the duty to put on a show but sometimes it's not possible," the Portuguese international, who joined Real from United for a record fee this year, told Wednesday's Marca sports daily.
"We always try but at times the team are not playing well and the fans could give us more support," he added.
"The Manchester United fans, for example, were very intimidating for Liverpool or Manchester City in those matches. I think that if our supporters tried to do that it would be much better for everyone and would give the players a big boost."
Ronaldo returned to action in late November after two months out with an ankle injury and told Marca his fitness was improving all the time.
He also said he intended to win back his FIFA World Player of the Year award, which was given to Barcelona forward Lionel Messi this month.
"I have the ambition to be the best," Ronaldo said. "By God's grace I have already won the trophy for the best player in the world but I hope to win it again, either this year or next, but I will win it again."