The Frenchman was replaced on the half hour, favouring on side with an apparent limp and knee concern.
“It doesn’t seem like anything serious,” Ancelotti said.
“Tomorrow they will do more tests. It could be that he is just suffering some discomfort.
“It’s unclear whether it’s a muscle issue or if it’s his knee.”
Benzema was critical to Real’s Champions League success last term, scoring 15 goals in the competition.
In his absence, Real went on to score three second-half goals from Vinicius Junior, Luka Modric and Eden Hazard.
“The team played very well in the second half and I’m extremely satisfied,” Ancelotti said.
“To be honest, the second half was an exhibition. We’re leaving with a clean sheet but I’ll take away how we played all round in the second half.
“We kept possession well and when we got some space, we used it brilliantly.”
Eder Militao was also subbed out with an injury concern, going off at half-time for Antonio Rudiger.
Ancelotti said: “He had a muscle overload in his hamstring and I preferred to avoid taking any risks, despite the fact that he was keen to carry on and also because Rudiger is very reliable.”