The top scorer in LaLiga this season with 22 goals, Benzema will not be available to face the Blaugrana at Santiago Bernabeu.
The France international netted twice in last Tuesday's (AEDT) 3-0 victory over Real Mallorca, but limped off towards the end of the contest indicating a problem with his calf.
Ancelotti, who is also without Ferland Mendy, revealed that Benzema will also not link with France during the forthcoming international break.
Addressing the media during his pre-match conference on Sunday (AEDT), the Italian said: "Both [Benzema] and Mendy can't play and won't go to France either. They stay in Madrid.
"He hasn't trained with the group. He still has some discomfort.
"Karim is a very important part, because he finishes all the work, but without him, you have to keep that work and look for different solutions.
"It is already decided who is going to play [in his place]. But I'm not going to tell you."
Benzema's league goals have come at a rate of one every 93.55 minutes, while he also leads the assists standings with 11 to his name already.
Asked if he is concerned by Benzema's latest lay-off, Ancelotti said: "He is a player who is 34 years old and sometimes it can happen to him.
"They are very minor annoyances and he has recovered very quickly. When he has come back, he has made a difference.
"We have another two weeks to work with him and then he will make a difference again.
"It doesn't worry me that he won't play tomorrow because we have time for him to play at the end of the season and make a difference."
Madrid sits comfortably top of LaLiga with a 10-point lead over second-placed Sevilla. Barcelona, in third, is 15 points adrift of Madrid with a game in hand.