The Portuguese was forced off with the injury in Real's 5-0 win over Almeria on Saturday and will now miss the chance to break the Champions League record for the number of goals scored by one player in the group stage.
Ronaldo has found the net eight times in just four European appearances this season, tying the record held by Ruud Van Nistelrooy, Filippo Inzaghi and Hernan Crespo. However, Ancelotti is hopeful his star man will be fit to face Valladolid in La Liga on Saturday.
"Cristiano cannot play tomorrow because of a small injury. Tomorrow I think he will start training on the pitch again individually and we will see if he will be available for Saturday.
"It is just a strain, but he is very confident about being able to play on Saturday. We won't take any risks with him because afterwards we have a Copa del Rey match in which he is suspended, but if he is fine then he will play."
Ancelotti also indicated that Ronaldo's absence won't affect the positioning of Gareth Bale on the right of the Madrid attack, where he has began to find form with four goals in his last five games. "I don't think he will change. I prefer him on the right so that he can shoot. I prefer the wingers to be able to come inside."
Madrid need just a point to mathematically secure their place in the last 16 and ensure they finish top of the group. But having scored 22 goals in their last five games, Ancelotti wants to finish the group with a flourish.
"We have two games and we want to win them both. We are not going to make calculations of what we need to do. Tomorrow will be a very intense game because Galatasaray are playing to qualify, but we want to win the game."
As well as Ronaldo, Madrid will be without Fabio Coentrao, Sami Khedira, Raphael Varane and the suspended Luka Modric. Meanwhile, Ancelotti hinted that Xabi Alonso could be rested with Brazilian midfielder Casemiro being handed his first start of the season.