PHILADELPHIA -- It might have happened when he thrust out his right leg and tripped Joel Embiid in frustration. It might have been when he crashed hard to the floor after Embiid blocked his shot.

Pascal Siakam isn't certain.

The one unmistakable thing is that the Toronto Raptors will sorely miss their athletic forward and second leading scorer on Sunday if a calf injury keeps him out of the game.

Siakam is listed as doubtful with a calf contusion for what Kyle Lowry called a "must-win" Game 4 of their best-of-seven Eastern Conference semifinal playoff series against the Philadelphia 76ers.

The Raptors trail the Sixers 2-1 after a blowout loss on Thursday.

"It's weird," Siakam said before the Raptors practised Saturday at Wells Fargo Center. "I feel like the team can always count on me to be out there every single night. But things happen."

Siakam led the team in games played in a regular season that saw a revolving door of injuries, and he's played the most minutes per night through eight playoff games.

But even more than his minutes, Siakam hasn't shrivelled in this series like the majority of his teammates. He's been Toronto's second option behind Kawhi Leonard, averaging 22.9 points per game in the post-season.

The drop-off after those two? Picture an abyss.

Siakam had 20 points in 30 minutes on Thursday before he was called for a flagrant foul when he stuck his right leg in the path of fellow Cameroonian Embiid during the fourth quarter. Embiid's knee appeared to strike Siakam's calf. Siakam left the game moments later and did not return.

"(But) there was a lot of contact (all game)," the 25-year-old said. "I was on the floor a lot."

Raptors coach Nick Nurse was cautiously optimistic about Siakam's calf injury.

"We've still got 24 hours or so for it to play out. I don't think there is anything seriously wrong structurally that is ruling him out 100 per cent or we would be telling you he is out 100 per cent," a terse Nurse said.

He could start either Fred VanVleet, Serge Ibaka or Norm Powell in place of Siakam, he said, but "none of it is ideal matchup wise."

The Raptors are already missing a small forward in OG Anunoby, who had an emergency appendectomy two days before the playoffs tipped off.

"With OG out it's not the ideal position for us. We are a little thin there," Nurse said.

If there's any positive to fall back on it's the fact that the team played through a string of injuries plus Leonard's load management during the regular-season.

"It's just like every other time someone has been out this year whether it's Kawhi or Kyle or whoever," Nurse said. "We just got to plug in with what we got and probably introduce someone else into the series who hasn't played yet."

If the Raptors needed the rest of the roster to step up before, the situation is significantly more dire now.

Lowry, in particular, struggled in Game 3, shooting 2-for-10, including 0-for-4 from three-point range. Looking unsure with the ball in his hands, he passed up several more wide-open looks.

"I know I can put the ball in the hole, and going out there with that mindset is going to be huge (Sunday)," said Lowry, who has traditionally rebounded from bad games with good ones.

After half a dozen solid defensive outings, the Raptors' defence was awful from the get-go on Thursday.

Why?

"I don't know. I don't know," Lowry said. "We were not tough. We were not physical. I'm not saying we're going to come out there and try to throw punches. But we have to come out and be a little more tough with it.

"They're using their bodies. They're using their size. They're very aggressive in attacking the glass. They're very aggressive in attacking everything. We've got to go out there and match their aggressiveness."

Both Lowry, at six feet, and VanVleet, who's 5-11, were outmatched in size by the Sixers.

"It's not really affecting me," he said. "OK, I know Jimmy (Butler, at 6-8) is a little bit bigger, he's coming from behind, the rear-view. That doesn't matter. Game 4 will be a different game."

It's not the first time Lowry has faced bigger guys.

"Nope. It won't be the last. It won't be the last," he said.

A loss on Sunday would send the Raptors back to Toronto just one defeat away from elimination in a season that began with so much promise, and a roster that was rebuilt with a run to the finals in mind.

How big is Sunday's game?

"Big. Big. Huge," Lowry said. "We need it. It's a must win for us. We want to win it. We're going to go out there and be desperate and play as hard as we possibly can to win that game."

Game 5 is Tuesday in Toronto. Game 6, if necessary, would be back in Philly on Thursday.