By the end of the first period, the lead was gone. So was Forsberg, who missed the final 40 minutes with a concussion after absorbing a wicked hit from Washington's Alex Ovechkin.
Led by Ovechkin and Alexander Semin, the Capitals rallied to a 4-1 victory Saturday night, extending the Flyers' skid to a franchise-record tying six games.
Semin had two goals and an assist and Ovechkin added a goal and two assists for the Capitals, who have won seven of nine - including two victories over Philadelphia.
Forsberg's return from a four-game absence provided the Flyers with a momentary lift. After finally finding a skate to accommodate his surgically repaired right ankle, the veteran centre received an assist on a goal by Mike Knuble that put Philadelphia up 1-0 at 3:31 of the first period.
But a hit by Ovechkin late in the period sent Forsberg flying, and his head hit the knee of Washington's Dainius Zubrus as he tumbled to the ice.
"A little bang in the head. It doesn't feel good right now," Forsberg said. "I kind of knew Ovechkin was going to hit me. I wasn't knocked out. I remember everything, so it's not that bad."
But the loss of Forsberg spelled doom for the Flyers, who are winless this season when their captain doesn't play.
"The turning point of the game was the Forsberg injury," Capitals coach Glen Hanlon said. "I'm not in their locker room, but I can sympathize because you get this emotional lift when your leader is back and you're playing well and have a lead. Then he's gone and the game is tied up. That's a tough one to recover from."
Flyers coach John Stevens, noting that Forsberg received two penalties before departing, said the hard hit was simply part of the game.
"It's hockey. Peter had a couple of big hits himself in the first period," Stevens said. "It's two great players going at it hard."
Despite Forsberg's abbreviated return and the acquisition of defenceman Alexei Zhitnik, the Flyers dropped a sixth straight game for the second time this season. Each streak includes an overtime defeat.
Zhitnik, acquired earlier Saturday in a trade with the New York Islanders, was inserted in the starting lineup logged a team-high seven minutes, 44 seconds of ice time in the opening period.
Although he didn't score, Zhitnik played solid defence and was used on both special teams.
Semin assisted on Zubrus' first-period goal, then scored twice in the second period to put Washington ahead 3-1.
Ovechkin, who had a hat trick one night earlier in Atlanta, scored with 16:44 left to make it 4-1. He has 22 goals and 21 assists in 32 games.
Olaf Kolzig had 21 saves for the Capitals, 3-0 against the Flyers this season.
A breakaway goal by Semin gave Washington a 2-1 lead at 5:52 of the second period. Semin took a lead pass from Brian Pothier, skated down the right side, deked Flyers goalie Antero Nittymaki and scored on a wraparound.
Semin made it 3-1 with his 14th goal of the season, on a perfect centring pass from Ovechkin one second after Zhitnik left the penalty box.
Knuble's goal stood up until late in the first period, when Zubrus flipped the puck past a prone Nittymaki to end a scramble in front of the net. Semin and Ovechkin received assists on the play.
Notes: Capitals D Jamie Heward was a healthy scratch; RWs Richard Zednik (groin) and Matt Bradley (hand) were also scratched. ... The Flyers gave up D Freddy Meyer and a conditional draft pick to get Zhitnik, a 14-year NHL veteran. ... Washington has outscored its opponents 12-4 in the final minute of a period.