Real surrendered top spot to their arch rivals when they were beaten 3-2 at Sevilla in their last outing and defeat to the champions would leave them six points adrift after 12 matches.
Benzema, one of several key players to have been sidelined in recent months, picked up a hamstring strain playing for France in October and has missed four domestic league games and two Champions League matches.
Benitez told a news conference on Friday Benzema was in the squad and a decision on his participation would be taken on Saturday.
Centre back and captain Sergio Ramos, who has been struggling with a shoulder injury that kept him out of the most recent Spain squad, would definitely play, Benitez added.
The former Valencia, Liverpool, Chelsea, Inter Milan and Napoli manager was also quizzed about Benzema's mental state following last week's attacks in Paris and his implication in a blackmail probe.
The 27-year-old was put under formal judicial investigation this month in connection with an alleged attempt to blackmail Olympique Lyonnais forward Mathieu Valbuena with the use of a sex video.
"We'll talk to him and assess how he is in every sense and decide what is best for us and him," Benitez said.
Real's Portugal forward Cristiano Ronaldo has been off colour in recent weeks and Benitez said he was sure the FIFA Ballon d'Or holder would be back on song soon.
"Cristiano is a fundamental player," Benitez, who will take charge of his first 'Clasico' at the Bernabeu, told reporters.
"He has the capacity to score a lot of goals, he has my absolute confidence that he will score a lot of goals and he is a guarantee and he will do that.
"However he does it I don’t care. The important thing is that he feels comfortable, that he gets forward and scores."
(Editing by Toby Davis)
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