The forward -- who was hurt after scoring twice in Madrid's 3-0 win at Mallorca on Monday -- was unable to train with his teammates on Saturday, in their last session before the high-profile fixture.
Benzema, 34, is LaLiga's top scorer with 22 goals, as well as providing 11 assists, and his absence is a blow for Madrid as they look to close in on the title.
"We never take risks," coach Carlo Ancelotti said in a news conference on Saturday. "Tomorrow [Benzema] can't play because he hasn't trained, he feels some discomfort. It's a small injury... [Gareth Bale] is one of the alternatives. We have Mariano [Diaz], we have [Luka] Jovic, I've played Isco and Rodrygo there too. There are a lot of options."
"Karim is 34 years old, it can happen," he added. "It doesn't worry me, we have two weeks now to work with him. We have time to get him ready for the end of the season, and he'll come back and make the difference like always."
Madrid go into El Clasico top of the table, 15 points ahead of Barcelona, and have won their past five games in all competitions.
However, Barca's form has also significantly improved under coach Xavi -- after a rocky start to the season with predecessor Ronald Koeman -- and have not lost in LaLiga since December.
"The identity of [Barcelona] won't change, it never does," Ancelotti said. "They have a clear style of play. Xavi is doing well. The team has improved a lot and they're in good form right now... We'll have to play an all-round game: defend well, press well, play well with the ball, bring it out from the back, be direct. A complete game."
The last time the two sides met, in January's Spanish Supercopa semifinals, Madrid won 3-2 after extra time.
Madrid will also be without left-back Ferland Mendy, who picked up a hamstring injury against Mallorca, although winger Rodrygo, who had to be carried off during the same game, trained on Saturday and is expected to be available.
Madrid's confidence was buoyed by their dramatic 3-1 comeback win against Paris Saint-Germain in their Champions League round-of-16 second leg last week, in which Benzema scored a 17-minute hat trick.
As part of their preparations for the World Cup in Qatar later this year, defending champions France will take on Ivory Coast in Marseille on March 25. They will then play South Africa in Lille four days later.
This is the first time that France coach Didier Deschamps has selected Giroud since the European Championship last year. He has scored 46 goals in 110 appearances for France.