Flyers general manager Paul Holmgren said Tuesday that Giroux's symptoms have gotten worse since he suffered a head injury against Tampa Bay last week.
"He skated a little bit today; didn't feel that good. Just didn't feel like himself," Holmgren said Tuesday night in Washington before the Flyers faced the Capitals. "I don't know that there's any good news in this other than after 15 minutes or so, once he stopped skating, he did start to feel a little better."
The immediate treatment will be rest.
"We're just going to take it easy with Claude," Holmgren said. "We'll probably just give him some time off (Wednesday) and revisit again on Thursday. Obviously we're concerned. Claude is one of our better players and obviously is off to a great start this season. We're going to err on the side of caution."
Giroux leads the NHL with 39 points and has led the Flyers to the top of the Eastern Conference standings.
Giroux took an inadvertent knee to the head by teammate Wayne Simmonds at center ice in the last minute of the second period on Saturday. He did not return for the final period.
The Flyers said on Monday that Giroux had been feeling better. But another examination Tuesday found Giroux has a concussion.
Giroux leads the Flyers with 16 goals and 23 assists, and his 39 points were three ahead of Toronto's Phil Kessel for the NHL lead.
Holmgren said on Monday that, "Claude is continuing to feel better," only to have another examination lead to worse news.
He's not the only Flyer suffering from a head injury. Flyers defenseman Chris Pronger also is sidelined indefinitely with concussion-like symptoms. Flyers forward Brayden Schenn also is out with a concussion.
But Philadelphia entered Tuesday night's game with the most points in the Eastern Conference.
"We're missing Claude and Chris Pronger. It's pretty hard to replace those guys," Holmgren said. "But other guys have played better."
Giroux's injury was revealed only a day after Pittsburgh Penguins star Sidney Crosby said he was dealing with a recurrence of concussion-like symptoms and is out indefinitely.
Crosby, who missed more than 10 months after taking shots to the head in successive games in January, hasn't played since developing a headache following a 3-1 loss to Boston last week.
The 24-year-old former MVP passed an ImPACT test - designed to diagnose concussions - last Wednesday but didn't travel with the team on a road trip through Philadelphia and New York over the weekend.