Gerrard sustained the injury in the 1-0 FA Cup fourth-round replay defeat to Everton last night.
“I don’t know how long Steven will be out for,” Benitez told the club’s official website.
“He was injured and he asked to come off. He will have a scan on Thursday and we will have to wait and see.
“I am really disappointed to have gone out of the FA Cup. To play for a long time with 10 players and then to concede a goal right at the end from a deflection, we have to be disappointed.
“The turning point of the game was the sending-off but I prefer not to say anything about that.
“You can lose players which is part of the game but when you play with 10 players and they are working as hard as they did tonight it was really bad luck at the end.”