Ducks right wing Jakob Silfverberg beats Capitals goaltender Braden Holtby for one of his two goals in Anaheim’s 5-2 win. (Mark J. Terrill/AP)

The penalty box has been a bugaboo for the Washington Capitals this season, a place they can’t seem to stay out of no matter how much they lament their frequent trips there. And after Sunday’s third period started with the team down a man, Washington kept putting itself in that situation with terrible results.

The Capitals put the Ducks on three power plays in the first five minutes of the third period and Anaheim scored on two of them, turning a tied game into an eventual 5-2 blowout loss for Washington. The Ducks hadn’t scored more than two goals in any of their past six games, but their offense came alive against the porous Capitals.

“If you want to be successful in this league, you have to stay out of the box and play five-on-five,” center Nicklas Backstrom said. “That’s something we’ve been talking about for a long time, and there’s no secret. I mean, I feel like when we play five-on-five, we’re a good hockey team. That’s the way we’ve got to do it.”

But Washington’s most significant loss might have come in the second period. After skating three shifts coming out of first intermission, forward T.J. Oshie was ruled out for the rest of the game with an undisclosed “upper-body” injury. Oshie has had five concussions in his career — including one earlier this season — but this wasn’t a head injury, though it’s unclear exactly what the injury is. Entering Sunday night, Oshie had scored five goals with 11 assists in his past 13 games, one of Washington’s most productive players of late.

Coach Todd Reirden said Oshie will be reevaluated on Monday, before the team plays the Los Angeles Kings. Asked if Oshie’s injury could keep him out long-term, Reirden said, “I don’t think so.”

With 27 seconds left in the second period, Capitals forward Tom Wilson was called for goaltender interference, but Wilson immediately objected that Ducks forward Corey Perry had pushed him into Ryan Miller. Regardless of if he had a case, Washington started the third period shorthanded and Adam Henrique scored his second goal of the game on a goalmouth scramble 37 seconds in. Washington has taken the second-most minor penalties in the NHL this season with 211 through 59 games.

“There’s some things that you see differently than the refs, but that’s the way it goes,” Reirden said. “You’re going to like some and then not like some others. It’s up to us to either take advantage when we get a break on the power play or do the job on the [penalty kill] when something doesn’t go our way. Special teams are always an important part of having success, and the details of the game for us weren’t as sharp.”

Capitals forward Dmitrij Jaskin was then assessed a tripping minor 3:02 into the period and the Ducks again scored shortly into their man-advantage, Perry doing the honors this time. Anaheim had scored just eight power-play goals on home ice all season before Sunday. Defenseman Matt Niskanen was called for tripping for another minor penalty less than a minute after that goal. Washington successfully weathered that penalty kill, but the damage was already done. Jakob Silfverberg’s second goal of the game rounded out the scoring for the Ducks with 5:05 to play.

But the Capitals felt like their game had deteriorated even before the third period started.

“We were probably fortunate to be tied going into the third there,” defenseman Brooks Orpik said. “I mean, it’s easy to point to special teams and penalties, but there’s a lot that went on before that. We just weren’t very sharp. Something we talked about before the game was that we’d been a lot better at minimizing the odd-man rushes we were giving up, and then we give up three in the first five minutes. We weren’t real sharp in a lot of areas.”

Among the issues was faceoffs, of which Washington won just 30 percent, chasing the puck for most of the night. The Capitals are the league’s worst team in the dot this season, and after captain Alex Ovechkin scored his 40th goal of the season less than five minutes into the game, the Ducks were able to tie it with a won draw that went to defenseman Brandon Montour. Henrique tipped Montour’s point shot past goaltender Braden Holtby 6:26 into the second period.

The Ducks have been one of the worst teams in the league this season. They dismissed Randy Carlyle as coach a week ago, with General Manager Bob Murray taking over behind the bench for the rest of the campaign. That’s part of what has made the Capitals so frustrating: Three nights after Washington had one of its most complete performances in a 5-1 win against San Jose, one of the NHL’s top teams, came this disappointing outing against lesser competition.

“There’s going to be games you lose, but if you’re play consistently and doing the same thing over and over, there’s going to be some nights you don’t get the results but you’re happy with the effort you put in,” Orpik said. “Right now, I think inconsistent is a good word to describe it.”