Benzema, who picked up the injury in Tuesday's 0-0 Champions League quarter-final first leg draw at Atletico Madrid, should be fit for the return game at the Bernabeu on Wednesday, Ancelotti told a news conference.
"Benzema is not available (for Saturday) but we will have him back for the next game," added the Italian.
Losing their first-choice striker, even if just for one match, is a blow to the European champions at a crucial stage of the season.
Benzema, a France international, has scored 15 goals in La Liga and six in the Champions League this term and is important to the team for his link-up play with fellow forwards Cristiano Ronaldo and Gareth Bale.
Ancelotti is likely to bring Isco into the starting lineup on Saturday, with the Spain midfielder operating in a central playmaking role alongside James Rodriguez.
Second-placed Real are two points behind La Liga leaders Barcelona, who host fourth-placed Valencia earlier on Saturday.
(Additional reporting by Silvio Castellanos, editing by Ed Osmond)
