Capitals defenseman Dennis Wideman upends Bruins winger Milan Lucic during the first period. (Ann Heisenfelt/AP)

The Washington Capitals suffered a momentum-halting 4-1 loss to Tim Thomas and the Boston Bruins on Sunday afternoon. It’s also possible they absorbed yet another injury to a key player.

Versatile forward Brooks Laich limped out of Verizon Center on crutches and sported an immobilizer on his left knee after getting crunched along the end boards in the second period by Bruins defenseman Dennis Seidenberg.

“I’m going to get some ice on it and see how I feel when I get up,” said Laich, who has 10 goals and 28 points, the sixth-highest total on the team. “I’m not a doctor, but with these things sometimes there’s some swelling and you’ve got to wait to see what happens. But I don’t think it’s anything serious. In the past I’ve been a fast healer. We’ll see when I wake up.”

While there probably won’t be an update on Laich’s condition until after Monday’s practice, at the earliest, this much is clear: The Capitals cannot withstand another injury, not with first-line center Nicklas Backstrom sidelined indefinitely with a concussion and No. 1 defenseman Mike Green recovering from abdominal surgery.

But that’s the possibility they’re facing, a reality made harsher after blowing their chance to leapfrog past the idle Florida Panthers and into first place in the Southeast Division on Sunday. They sit ninth overall in the Eastern Conference, one point behind the Panthers and two points behind eighth-place Toronto.

Even before Laich got hurt, the Capitals (27-21-4) were having a rough afternoon, particularly on defense.

Midway through the first period, Milan Lucic gave the Bruins (33-16-2) a 1-0 lead after splitting defensemen Jeff Schultz and Dennis Wideman. With Lucic behind Wideman on the rush, Rich Peverley threaded a pass to the bruising Bruins winger, who fired the puck past Tomas Vokoun.

“No miscommunication, I just lost Lucic,” Wideman said of the miscue. “I let him get in behind me and he made us pay.”

Then, with 82 seconds left to play in the opening period, Brad Marchand sneaked into the slot and fired a pass from Patrice Bergeron into the net. Bergeron had wrestled the puck away from Karl Alzner along the end boards before shoveling the puck past John Carlson to an opportunistic Marchand.

“It’s D-zone coverage and you have to make sure you have your guy,” Capitals Coach Dale Hunter said. “Unfortunately, if you miss your guys for a second, it could be in the net and that’s what happened.”

The 2-0 lead was all Thomas needed. Even though Alex Ovechkin launched a game-high seven shots on net and the Capitals outshot an opponent for the first time in 14 games, the reigning Vezina Trophy and Conn Smythe winner shut down the hosts. In fact, Washington’s only goal, scored by Marcus Johansson late in the third period, went into the net after it deflected off of Seidenberg’s skate.

“Tim’s capable of doing that for us,” said Boston Coach Claude Julien, whose team was outshot, 36-29. “I thought he was on top of his game today.”

Thomas’s heroics, though, were the least of the Capitals’ worries after the game. There was obvious concern about Laich, whose legs got sandwiched against the boards by Seidenberg on a clean hit.

Laich, who is second among forwards on the Capitals in ice time at 19 minutes 13 seconds per game and can be used in virtually every situation, remained on the ice and eventually needed assistance from head athletic trainer Greg Smith and Alzner to make it to the bench. Laich attempted to return to the game moments later, but he was unable to withstand the discomfort.

Despite the injury, Laich maintained his trademark sense of humor. He joked that the injury was to his elbow and that he was headed home to do some squats. He also tried to convince reporters he was fine by taking a step without assistance from his crutch.

“A lot of times, if you block a shot you get a stinger,” said Laich, who has not missed a game since March 2010. “I thought that’s maybe what it was, that I hit a nerve or something and it was a bit of stinger.”

After Laich left the game, the Bruins took a 3-0 lead on a redirection off Tyler Seguin 6:38 into the third period. Moments after Seguin’s strike, Capitals rookie defenseman Dmitry Orlov was struck under the visor — the same place where Orlov was struck by a puck Saturday in Montreal — by a Shawn Thornton shot that deflected off teammate Roman Hamrlik’s stick.

“He broke his nose last game, so he got another puck there,” Hunter said. “He’s fine. But his eyes were watering too much to play.”

Hunter was less certain about Laich.

“We’re going to see tomorrow,” he said. “He’s day-to-day right now. Brooksie is one of those guys that try to do anything for the team.”