The Portugal international will miss Wednesday's encounter due to a thigh injury, while the Madrid coach has stressed that the Turkish champion should not be underestimated.
The Portuguese attacker picked up a thigh problem in Saturday's 5-0 win over Almeria and was withdrawn nine minutes into the second half.
Ronaldo had scored his 25th goal of the season for Madrid in just the third minute, but will be unavailable for this week's Champions League clash against the Turkish champion, with Angel Di Maria and Jese vying to be his replacement.
"Cristiano won't play tomorrow because he has a small injury. We will need to wait and see if he can play on Saturday," Ancelotti stated at a press conference. "Angel Di Maria or Jese Rodriguez could replace Ronaldo. We have two options and I will decide on Wednesday.
"We won't risk and play Ronaldo on Saturday against Real Valladolid unless he is 100 percent fit."
The Madrid coach then went on to insist that Galatasaray will not be a pushover in Wednesday's encounter at the Santiago Bernabeu.
Real Madrid tops Group B and has already secured a place in the last 16 while Galatasaray still has everything to play for, being in a three-way tussle with Juventus and Copenhagen for second place.
"Galatasaray played well against Juventus. They are well-balanced and have dangerous players like Didier Drogba," he continued. "Roberto Mancini has a lot of experience. He has done a great job in Italy and England where he won the Premier League.
"We want to qualify as group leaders. It will be an intense match."
Madrid beat Galatasaray 6-1 in Istanbul earlier this campaign.