Joffrey Lupul's status is now the biggest question mark surrounding the club as the Maple Leafs continue the push for their first playoff berth since 2004.
Former Toronto tough guy Jay Rosehill scored the winner in his first game with Philadelphia and was also involved in the play that knocked Lupul out of action as the Flyers downed the Maple Leafs 5-3 for their fourth straight victory.
The red-hot Lupul, who assisted on the first Toronto goal and has 14 points since returning from a 25-game absence because of a broken forearm, appeared to take a blow to the head in the first period after being sandwiched by Rosehill and Adam Hall, who also made his debut for Philadelphia.
The Maple Leafs sniper got back to his feet as play continued but was wobbly and had to be helped onto the bench. He left for the locker-room shortly afterwards and did not return because of an upper-body injury.
"You always take those situations with caution and he feels fine now, but that's what happens in the context of the game," said Maple Leafs coach Randy Carlyle, whose team saw its three-game winning streak and eight-game point streak snapped. "We'll have a much better read (on Friday). If we're to say that he's probably going to practise (Friday) I would say that he's 50-50 on practising."
Without knowing the extent of Lupul's injury, Toronto goalie James Reimer expressed concern for his teammate.
"He's done a lot for us. He's a great player. I don't know how long he's out for but hopefully it's short. But if it's a couple games or a couple weeks, we've got to step up in here," said Reimer, who finished with 26 saves in suffering just his fifth regulation loss of the season. "He's a huge boost for us but we do have guys that can step in and fill that role and play those minutes."
Simon Gagne, Jakub Voracek, Brayden Schenn and Luke Schenn, into an empty net against his former team, had the other goals for Philadelphia (17-17-3), which is fighting for its playoff lives.
"We didn't play desperate enough. We didn't execute to a high enough level. We were a step behind," said Carlyle, who saw his team drop down to sixth in the Eastern Conference behind idle Ottawa. "Our execution was extremely poor."
Sean Couturier added two assists and Ilya Bryzgalov made 25 saves for the Flyers, 11th in the conference, two points back of eighth.
"I really like the way we're playing right now. You could just tell that we were going to go out there and work hard tonight," Flyers coach Peter Laviolette said. "The new guys that came into the lineup were terrific in that regard."
Nikolai Kulemin, John-Michael Liles and James van Riemsdyk, with his second of the season against his former team, scored Toronto (20-13-4). Dion Phaneuf chipped in with two assists.
Carlyle grouped Reimer in with the rest of the Maple Leafs when assessing his team's lacklustre play. The netminder was given a vote of confidence a day earlier when Toronto management decided against bringing in a veteran at the NHL trade deadline.
"We weren't as sharp as we needed to be to have success," said Carlyle, whose team was coming off a four-day break following Saturday's emphatic 4-0 shutout of the Senators in Ottawa. "You score three goals in the NHL, you should win."
Tied 2-2 after an eventful first period, Brayden Schenn gave the Flyers the lead just 54 seconds into the second on a terrible giveaway by Kulemin in the Toronto zone. Schenn moved into the high slot and fired a shot that squeaked through Reimer.
"I got most of it, just not enough," Reimer said. "You'd like to make a big save there but it wasn't in the books."
Philadelphia then stretched its lead to 4-2 at 15:53 when Couturier's shot from the point was tipped in by Rosehill, who was playing his first game with the Flyers since being acquired from the Anaheim Ducks on Monday.
Reimer complained to the officials that he was interfered with, and replays suggested he had a case, but the goal stood.
"I was just trying to come in here and make an impact and have a good start to being in this organization," Rosehill said. "I'm pretty happy with the way it went. A big win."
Van Riemsdyk brought the Leafs closer at 6:14 of the third with his first goal in 11 games, roofing a shot from in tight on Bryzgalov for his 15th of the season.
Toronto had another couple of chances to tie the score as the period wore on, including a big push in the final minutes, but couldn't get the equalizer before Luke Schenn scored into the empty net.
Gagne opened the scoring on a nice deflection past Reimer just 1:19 into a physical first period before Kulemin tied things up at 3:22, banging a loose puck past Bryzgalov.
Toronto went ahead at 16:46 on a shot from the point from Liles that found its way past Bryzgalov thanks to a Frazer McClaren screen.
That lead was short-lived as Couturier found Voracek on a great pass and he swept the puck past a helpless Reimer for his 16th goal of the season at 18:35.
"Shift after shift nobody wants to let each other down with the work ethic right now," said Laviolette, whose team beat the Montreal Canadiens 5-3 a night earlier. "It was a good win. It was a good win on the back-to-back."
Notes: Luke Schenn and van Riemsdyk were traded for each other in the off-season. ... The Flyers acquired former Calder Trophy winning goalie Steve Mason from the Columbus Blue Jackets prior to Wednesday's trade deadline for backup goaltender Michael Leighton and a third-round pick in the 2015 draft. Mason wore No. 35 for Philadelphia ... Bryzgalov has played in 35 of 37 games this season. ... Maple Leafs defenceman Ryan O'Byrne, acquired from the Colorado Avalanche on Wednesday, arrived in Toronto in time for the pre-game warmup but did not dress. ... The Flyers are in Winnipeg to take on the Jets on Saturday, while the Maple Leafs visit the New Jersey Devils. ... Attendance at Air Canada Centre was 19,619.