MICHAEL DEL ZOTTO was finishing a routine, three-on-three battle drill below the circles in practice yesterday with partner Carlo Colaiacovo. He fired a shot on Steve Mason and turned around to skate out of the zone so the next group could enter the drill.

It never resumed. Everyone on the ice was gasping.

Mason, the Flyers' backbone, was face-down on his stomach and unable to move with an apparently serious injury. Some players described it as "urgent."

Neither Del Zotto, nor Colaiacovo, or any other player on the ice bumped Mason. There was no apparent awkward movement or twist or turn.

"I don't know what happened," Del Zotto said. "I saw him go down and that was it. I turned for the next group to go in and I'm not sure what happened after that."

The Flyers' mood shifted in one seemingly random sequence and practice abruptly ended.

"I was kind of shocked," Mark Streit said. "It's just a shock for the team. He's been the backbone of this team. He's been playing really well for us."

Mason did not accompany the team on its flight to Toronto 2 hours later, and so the Flyers began a season-long eight-game road trip against the Maple Leafs tonight without him.

According to general manager Ron Hextall, Mason is ruled out for at least this weekend's games in Toronto and Winnipeg with an "upper-body" injury, but the team would not commit to any timeline after that.

That doesn't necessarily mean Mason will miss only this weekend. Hextall said the team will provide an update on Monday and would not comment further.

The scene yesterday was ominous. With the help of the training staff, Mason was flipped over to his back - where he stayed for a period of time - before sitting up and finally making it to his feet. He was then helped off the ice, unable to skate off under his own power.

The Flyers did not open the team's dressing room to media until after Mason was helped out of his equipment. His pads were scattered in a pile on the floor.

R.J. Umberger said Mason's injury "looked like" a back issue, noting Mason "grabbed something behind him" when he "reached for a puck" to make a save.

Luke Schenn saw Mason go down and had his own theory.

"It looked like he was going into a spasm or whatever it was," Schenn said. "I'm not even sure. You never know if it's dehydration or anything like that.

"He went down to make some sort of save and he locked up or something. I have no idea. He obviously faced a lot of shots [Thursday night], it was a long game, it was hot [inside Wells Fargo Center], there was a shootout.

"I'm thinking [it could be] dehydration. That's at least what I'm hoping. I think that's the best-case scenario. We're really hoping it's not too bad. He's been unbelievable for us."

Mason has a limited injury history with the Flyers. He has, however, been prone to cramping. During timeouts in some games, Mason has returned to the bench to gulp down a banana and water to alleviate cramping symptoms.

This also would not be his first bout with back spasms, if that were the case. According to the Columbus Dispatch, Mason's back acted up on multiple occasions during his rookie season in 2008-09 with the Blue Jackets, including one notable spasm.

Mason, 26, has compiled a .921 save percentage and 2.49 goals against-average this season. His numbers, aside from his record, are pretty impressive considering his slow start to the season and the lack of defensive support he's had in front of him.

Backup Ray Emery is expected to start tonight. Veteran goaltender Rob Zepp was called up from Lehigh Valley to replace Mason on the roster. He could well make his NHL debut at some point on the road trip.

Zepp, 33, is in his first year back in North America after nearly a decade in Europe. His road to the NHL has been long and winding, having now outlasted all but five of the 98 players drafted ahead of him in 1999. He is 8-5-4 as the Phantoms' starter with a 2.70 goals against-average and .918 save percentage this season.

Mason's injury comes at just about the worst possible time. After losing two straight, the Flyers entered last night eight points back of both a wild-card spot and third place in the Metropolitan Division with a staggering eight teams to jump over.

"Things happen sometimes," coach Craig Berube said. "It's devastating to lose players, any player. It's a big, big road trip. We need points. I don't know what happened, to be honest with you, I was more watching the guys with the drill we were doing. I just kind of looked over and Mason was on his stomach."