The former Manchester United forward picked up a strain in his left thigh during Madrid's 5-0 win at Almeria on Saturday, forcing him to be substituted in the 54th minute.
The Madrid coach reiterated that the injury was nothing to be worried about and was hopeful he would only miss the match with Roberto Mancini's side and could be back in time to face Real Valladolid at the weekend.
"Cristiano can't play tomorrow, he's had a very small problem but tomorrow we're going to rest him and we're going to see if he can be available for Saturday," Ancelotti told a press conference.
"He has a small muscle strain but he is very confident he can play on Saturday, but if there's any risk of him getting injured, he won't play."
Ronaldo has scored 14 goals in his last seven matches for club and country and Madrid fans inside the Bernabeu will pay tribute to the forward by wearing 45,000 masks of his face to show their support for his bid to win the FIFA Ballon d'Or.
Ronaldo was replaced by 20-year-old Jese Rodriguez against Almeria and Ancelotti said either the youngster or Angel di Maria would start on the left of attack in the Portuguese's place.
The coach also confirmed he would not change Gareth Bale's position on the right of Madrid's attack in Ronaldo's absence.
"Bale plays on the right to take advantage of his shooting ability, I prefer my players to play on a different side to their stronger foot," he said.
Ronaldo, Sami Khedira, Raphael Varane and Fabio Coentrao have all been ruled out through injury, while Luka Modric is suspended after picking three yellow cards in the first four Champions League games.
Varane has struggled to remain match fit after undergoing knee surgery last May, starting a total of just six games this season, and Ancelotti revealed he would be rested for up to a fortnight to prevent the knee from becoming inflamed.
Another player who could miss out is Xabi Alonso, who returned to action less than a month ago following a groin operation and then a fractured metatarsal.
"We have to be careful with Alonso because he's just come back from an important injury, playing two games in four days could be a problem, so there's a chance he will rest and Casemiro will play instead," explained Ancelotti.
Madrid romped to a 6-1 win over Galatasaray when the two teams met in September but Ancelotti believes the Turkish champions are a stronger side under Mancini, who succeeded Fatih Terim as coach a week after that match.
Madrid have taken 10 points from their four Champions League matches and just need a draw to mathematically ensure they qualify as group winners, but just a point separates second place Galatasaray from bottom side Juventus.
"It will be a different game to the one in Istanbul, Galatasaray have changed a lot since Mancini arrived," said Ancelotti.
"Now they have more balance and have quality in attack, with players like Didier Drogba. We expect a very difficult game because they are playing for qualification."