Giroux was injured in the second period of the game Saturday when the knee of teammate Wayne Simmonds hit Giroux in the back of the head.
Giroux was low on the ice trying to make a defensive play and Simmonds, trailing the play, tried to jump over him but hit Giroux with his right knee.
Flyers General Manager Paul Holmgren Monday said Giroux "is continuing to feel better" but wouldn't make the trip for Tuesday's game in Washington. Tuesday he had a much different update.
"Claude reported not feeling very good today," Holmgren said. "Over the past few days his symptoms have gradually gotten worse. He will be out indefinitely with a concussion."
Giroux, 23, has 16 goals and 23 assists for a league-leading 39 points this season. He led the Flyers last year with 76 points.
The Flyers have two other players -- team captain Chris Pronger and rookie Brayden Schenn -- out due to concussions.