The Portuguese has struggled with injuries since the back end of last season and has also not been called up by Portugal for the upcoming international break, which means almost three weeks off for the mercurial 29-year-old.
"He is okay but we have preferred to use this week and the following two for him to recover fully and get him in top condition. He will not play tomorrow and is not in the squad," Ancelotti told a news conference in Madrid.
Ronaldo has recovered from the knee injury that affected him at the end of the last campaign but was also not 100 percent at the World Cup in Brazil.
Of late, he has been troubled with a hamstring strain he picked up in the defeat by Atletico Madrid in the Spanish Super Cup.
His absence means James Rodriguez is likely to play in the three-man attack alongside Gareth Bale and Karim Benzema with Isco slotting into the midfield.
Real have been weakened by the departures of Angel Di Maria and Xabi Alonso this week, and Ancelotti admitted he was taken aback by the decision of the latter to go to Bayern Munich.
"It was a surprise. Everything was calm and so we were a bit surprised but we respect him a lot. Alonso is a very honest person with himself and with the Real Madrid," said the Italian.
"He explained it well and said that he needed more motivation. Everyone here at the club respects him and it would have been very difficult to get him to change his mind. You can't say to a player like Xabi that he couldn't leave. I wish him luck and he has been a very important player for the team last season.
"It is possible that some people think that the team is under-strength but we have players that can perform in different positions. We have five strikers because Isco and James (Rodriguez) can play there.
"The squad is not closed because the market is still open. Maybe we will sign somebody but at the same time it is possible we won't."