The Flyers traded their captain before the game and then lost 4-2 to Toronto on Thursday night.
Mats Sundin had a goal and an assist to help the Maple Leafs snap a three-game losing streak.
Forsberg was pulled from the ice in pre-game warmups and traded to the Nashville Predators for right wing Scottie Upshall, defenseman Ryan Parent, and two draft picks.
The Maple Leafs scored three goals in the first period and chased Philadelphia goaltender Michael Leighton, who had won his previous two starts.
"It's very difficult to have your guy leave the ice and be traded," Toronto coach Paul Maurice said. "It's tough and I thought they did a great job of kind of battling back after the first and regrouping as a team and come out and play real hard."
"I don't think there's anybody that can survive at least emotionally and be ready in the first period when something that drastic happens to your hockey team," Maurice said.
Toronto goaltender Andrew Raycroft stopped 34 shots to pick up his 26th win.
O'Neill scored through a screen from the right circle at 6:39 and Antropov added another goal from in close on a power play at 8:54.
O'Neill said that the Forsberg trade definitely affected the Flyers.
"It's got to be pretty deflating when you're taking the ice and your best player just left the team," O'Neill said. "I think that probably was a factor in their slow start."
Leighton, who faced 13 shots, was replaced by Antero Niittymaki at 14:10 after Colaiacovo jumped out of the penalty box, skated, in and scored on a wrist shot.
Gagne got one back for the Flyers on a power play at 12:10 of the second period when he scored his 30th goal of the season on a shot from the right side.
Gagne said that the team learned of the Forsberg trade right before the game.
"You could see that Peter's skates and equipment was not there," Gagne said. "It was not easy to go out and play that game right away."
Sundin made it 4-1 at 11:46 of the third period when he lifted a shot high over Niittymaki from inside the right circle.
Sundin said that he was "not surprised" by the Forsberg trade.
"I think the year Philadelphia has had and with Peter at this stage of his career, it's good for both Philadelphia and for Peter to have a solution this way," Sundin said.
Knuble got the Flyers final goal from in close on a power play three minutes later.
"It was surreal," Knuble said. "Five minutes before you go out on the ice, you can't play your game."
- Gagne has had three 30-plus goal scoring seasons.
- The Maple Leafs are 17-10-3 on the road this season.
- The Flyers have dropped four straight games to Toronto.
- Philadelphia outshot Toronto 16-3 in the third period.