Benzema scored twice in five second-half minutes to put the game beyond Madrid's host on Tuesday (AEDT), but he looked in some discomfort while celebrating Los Blancos' third goal.
Soon after, in the 85th minute, Benzema departed having indicated a problem with his calf, forcing Madrid to close out the match with 10 men as they had already used their five permitted substitutions.
The two goals moved Benzema's season tally to 22 in 25 LaLiga matches, and although Ancelotti admitted they are in "wait and see" mode, he does not expect anything serious.
A major problem for their key man would be a significant blow for Madrid ahead of Monday's El Clasico.
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"We have to evaluate him in the next few days – things don't seem very serious," Ancelotti said.
"Karim did not feel good after a jump – we have to wait until Wednesday [to learn more]."
Real Madrid now sit 10 points clear of second-placed Sevilla but face a serious test of their credentials next with the visit of a resurgent Barcelona.