The Portugal international will miss Wednesday's encounter due to a thigh injury, while the Real Madrid coach stresses the Turkish champions 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The Portuguese attacker picked up a thigh problem in Saturday's 5-0 win over Almeria and was withdrawn nine minutes into the second half.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 to face the Turkish champions, with Angel Di Maria or Jese vying to be his replacement.
"Cristiano won't play [on Wednesday] because he has a small injury. We will need to wait and see if he can play on Saturday," Ancelotti stated to the press.
"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 per cent fit."
The Madrid boss warned that Galatasaray will not be pushovers when they arrive at the Santiago Bernabeu.
Real Madrid top Group B and have already secured themselves a place in the last 16 while Galatasaray still have plenty of incentive, locked in a three-way tussle with Juventus and FC Copenhagen for second place.
"Galatasaray played well against Juventus. They are well-balanced and have dangerous players like Didier Drogba," the Italian 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."