The Portuguese playmaker was thought to be nearing fitness for the International Champions Cup clash in Michigan on Saturday, with Ancelotti refusing to rule him out on Wednesday.
But the Ballon d'Or winner's comeback to club duties after a failed FIFA World Cup campaign with Portugal in Brazil was put on ice, despite Ronaldo completing multiple training sessions.
The revelation will be a letdown for the predicted 100,000-plus crowd, but Ancelotti said he did not want to rush his superstar attacker - instead resting him for the UEFA Super Cup against Sevilla on August 12.
"Ronaldo has been working individually, separate from the team," Ancelotti said.
"So he will not play against United and will train when we get back to Madrid because we will use him in the Super Cup.
"The plan for Cristiano was right. We gave him one month of rest and he started back on July 26.
"But he needs to recover completely and, for this reason, we don't want to take a risk tomorrow."
On the rumours that linked him to the United job before Louis van Gaal was appointed at Old Trafford, Ancelotti said he never had any thoughts of entertaining a move to Manchester.
"These are rumours that usually happen, but I am at a fantastic club," Ancelotti said.
"I have lots of respect for United, but I am happy at Madrid and I think they have made a good chance in Louis van Gaal.
"He is a very good and very experienced manager.
"He has fantastic experience and he will do a fantastic job at United because he is a manager who has nothing to learn.
"After Sir Alex Ferguson, it was more difficult for United, but they now have to rebuild a team.
"They have signed good players, but building a team is not easy and the Premier League is a very competitive league.
"It will not be easy to win in his first year because there are many competitive teams who want to win."
