The Portugal captain and World Player of the Year has been hampered by a left knee problem in recent months and was clearly below his best at the World Cup in Brazil, where the Portuguese failed to progress beyond the group stage.
"Cristiano Ronaldo is doing individual work and the plan is continue with the programme and on Aug. 5 he will train with the team again," Carlo Ancelotti, the coach of the European champions, told a news conference previewing the match at the University of Michigan stadium.
Ancelotti added that he was counting on Ronaldo recovering in time for the European Super Cup against Real's La Liga rivals and Europa League winners Sevilla in Cardiff on Aug. 12.
The game against United is part of the International Champions Cup friendly tournament.
Real lost their opening two Group A matches against Serie A sides Inter Milan and AS Roma respectively and Ancelotti said he and the players were determined to put on a good show despite already being eliminated.
"There are never any friendly games against Manchester (United), and less so in a stadium like this," Ancelotti said.
"We have to finish this tour off in the best way possible," added the Italian.
"We are proud to play this match and we want to finish well before playing the Super Cup."
Real begin their La Liga campaign on Aug. 25 at home to promoted Cordoba.
The first leg of the Spanish Super Cup against city rivals Atletico Madrid is at Real's Bernabeu stadium on Aug. 19 and the return is at the Calderon on Aug. 22.