The Frenchman was replaced at half-time of Saturday's shock 1-0 defeat to Deportivo Alaves, which saw Madrid rack up a fourth straight match without a goal in all competitions.
Madrid coach Julen Lopetegui confirmed after the game that both Benzema and Gareth Bale suffered injuries as his side's winless run extended to four matches.
No timescale has been put on a return for Benzema, but early reports suggest he will be absent for two weeks.
Madrid travel to Camp Nou to face Barca on October 28, which would leave little time for Benzema to prove his fitness.
The striker has four goals this La Liga campaign, nearly matching his entire haul from the previous season, when he scored just five times in the league.
Benzema's medical report. https://t.co/lhfFOSCQ8s— Real Madrid C.F. (@realmadriden) October 8, 2018
Benzema has not been able to find the back of the net since, his fortunes falling in line with the rest of his team-mates who have not scored since a 1-0 win over Espanyol on September 22.
The striker did receive some good news on Monday however, as he was among the 30 nominees for the Ballon d'Or, with Benzema's team-mate, Luka Modric, the likely favourite for the award.