The Capitals fell to the Lightning 4-2 in Game 1 of this Eastern Conference semifinal series Friday night, going 0-for-5 on the power play and choosing to swap chances with their opponent in ‘river hockey’ style rather than stick to their system. What remains to be seen at this point, though, is whether the contest the biggest loss to come out of the evening.

Rookie John Carlson went off the ice wincing in considerable pain with 5 minutes and 23 seconds remaining in the second period after a collision with Tampa Bay’s Nate Thompson. It’s unclear what exactly hindered the defenseman enough to force him to miss all but 36 seconds of the rest of the game.

Carlson came out for a brief shift (20 seconds) at the start of the third period but went to the bench quickly and was attended to by the team’s athletic trainers. He skated during television timeouts and took to the ice for one final shift but returned to the bench once again, slamming his stick.

After the game, Coach Bruce Boudreau said Carlson, who has averaged 23:24 per contest in the playoffs, is day-to-day.

“I’m hoping he can go Sunday,” Boudreau said. “When you lose John you lose pretty well half of your offensive defensemen — him and Mike [Green]. The other guys are really good at defending but they’re not known for offensive prowess.”

Carlson has not missed a game this season and over the course of the year he has developed, along with his usual defensive partner Karl Alzner, into Washington’s shutdown pair.

In addition to playing over 23 minutes per game in the playoffs, Carlson, 21, averaged more than 22 in the regular season — the third highest on the team behind Green (25:11) and Dennis Wideman (24:04), who has been out since Mar. 29 after suffering a leg hematoma.

While the Capitals have withstood the loss of several key defensemen over the course of the season because of injury — from Tom Poti to Green and Wideman — should Carlson miss time it will undoubtedly have an impact. In the third period when the Capitals were down to five defensemen, Green played 11:25 of his team-high 27:05 in Game 1.

Alzner and Scott Hannan played 7:25 and 6:28 in the third period, respectively.

“Any time you lose a guy that plays the right side,” Hannan said, “He’s been playing a lot of key minutes for us, and it’s tough.”