The Frenchman had been a doubt all week but missed training on Saturday and is out of the fixture at the Santiago Bernabeu.
“Benzema and Mendy won’t be here tomorrow and they’ll stay next week working with us,” he told a pre-match press conference.
“We’re not going to risk it. He can’t play tomorrow, he hasn’t trained. He has discomfort and can’t train or play.
“I’m not worried about Benzema. He’s 34 so he has suffered small problems this season but he has always returned stronger. He will recover next week and he will be ready for the most important part of the season.”
Ancelotti also confirmed he’s already decided who will replace Benzema, but wouldn’t say who, and added that he’s feeling confident about his team’s chances even without the striker.
“We can beat Barça without Benzema. He is a very important part, he is a great finisher. We have to look for different solutions in attack without Karim,” he added.
“Bale is an alternative to Benzema, like Mariano, Jovic, Isco, Rodrygo. There are many options but I will choose the one I think is best for this match.”
There was also time for Ancelotti to talk about his opposite number and he did praise Xavi for the job he’s done at Barcelona since replacing Ronald Koeman.
”The identity of the team hasn’t changed, the style is very clear,” he said. “Xavi embodies it very well, he’s doing well, in a good dynamic. Barça is very complete and Xavi is doing very well.”
Benzema will be a big miss for Los Blancos. The 34-year-old has been in terrific form this season, scoring 22 times and contributing 11 assists in 25 matches in La Liga.