The Portuguese, who won 2014's Goal 50 on Monday, has been training with the Blancos squad in pre-season and there were suspicions he could be fit enough to face the Red Devils in the International Champions Cup clash this weekend.
However, Ancelotti has poured cold water on the notion by insisting that the 29-year-old, who has been struggling with fitness since April and did not look fully fit at the World Cup, would stay on the sidelines in the tie with Louis van Gaal's men.
"Cristiano Ronaldo has been working separately from the team," the former Paris Saint-Germain trainer told reporters.
"We won’t put him out tomorrow because he hasn’t been training with the full squad. He will begin training with the team when we return to Madrid as we plan to use him in the Supercopa.
“The plan for Cristiano was right. We programed good rest for him. He started on the 26th, but he needed to train individually so he could recover fully.
"For this reason we don’t want to take any risk with him for tomorrow. We will continue his program and he will rejoin team training back in Madrid."
Portugal revealed that their captain had tendinitis ahead of the World Cup in Brazil and he failed to shine as his team crashed out at the group stage.
Real Madrid's clash with Man Utd kicks off just after 10:00CET on Saturday.