The 29-year-old has been struggling with fitness in the past few months and his coach has confirmed that he will not be facing his former club Saturday.
The Portuguese, who won 2014's Goal 50 on Monday, has been training with the Blancos in preseason and there were suspicions he could be fit enough to face the Red Devils in the International Champions Cup clash this weekend.
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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 match against Louis van Gaal's men in Ann Arbor, Michigan.
"Ronaldo is working hard but we won't put him out tomorrow against Man Utd," Ancelotti told reporters.
Ronaldo missed much of the end of Madrid's 2013-14 campaign, including the Copa del Rey final, but did come back in the Champions League showdown with Atletico Madrid to help it to extra-time Decima glory.
Portugal confirmed that its captain had tendinitis ahead of the World Cup in Brazil, and he failed to shine as his team crashed out at the group stage.