John Tavares slides into the net with Capitals goalie Philipp Grubauer, who made 40 saves on 45 shots in his first NHL start. (Bruce Bennett/GETTY IMAGES)

The Washington Capitals had tied the game in the third period on Saturday afternoon, finally creating some momentum for themselves at sleepy Nassau Veterans Memorial Coliseum. But just as it looked like the Capitals might find a rhythm against the New York Islanders, they derailed their own efforts with penalties.

Of the final 11 minutes of a 5-2 loss, Washington spent 6 minutes 11 seconds on the penalty kill, thanks to a pair of minor offenses by Mike Ribeiro and a double-minor to Jeff Schultz. The only reason they didn’t spend more of that span short-handed was because Islanders star center John Tavares scored two decisive power-play goals.

The loss snapped the Capitals’ three-game winning streak and handed rookie netminder Philipp Grubauer a loss in his first career NHL start despite 40 saves.

The untimely and frequent penalties were a recurrence of what had been a key problem for the Capitals early this season. Recently they had been more disciplined, committing no more than four minor penalties in any of the six previous games.

“Hopefully it’s a one-game thing. It’s obviously very frustrating,” Coach Adam Oates said. “We got a big goal and penalties hurt us, simple as that. Four-minute penalties, two of them. Their power play is a good power play; we held them for a while. It hurts.”

The Capitals (10-12-1) meandered off course quickly after Nicklas Backstrom scored on a one-timer blast from the top of the left circle to even the game at 2 just more than seven minutes into the third period.

Ribeiro was whistled for high-sticking Tavares at 8:48 of the third, then earned a second minor for unsportsmanlike conduct after swinging his stick at the boards and voicing his unhappiness with the initial call to the referees. It marked Ribeiro’s third unsportsmanlike penalty of the season.

“Mike knows better than that,” Oates said. “We’ll talk. I’m sure he feels bad about it.”

The compounding penalties forced Washington to face the Islanders’ potent power play, which entered the game ranked sixth in the NHL with a 23.1 percent effectiveness, for far too long. With 35 seconds remaining in the second penalty to Ribeiro, an unguarded Tavares scored to make it 3-2 when a rebound from a Brad Boyes point shot popped out to him on the right side of the cage.

Penalties are “going to creep up on you once in awhile. The penalties that we took are penalties that could have been avoided,” forward Troy Brouwer said. “Nothing against Ribs, but it’s timing. Timing’s not good. I feel like we shot ourselves in the foot tonight. We were right there in the game, 2-2, and you know we took ourselves out of the game with turnovers.”

Schultz went off for four minutes after high-sticking and cutting New York defenseman Andy MacDonald all of 22 seconds after Tavares’s first goal. Eight seconds into that power play, Tavares made it 4-2. His shot ricocheted off the goal camera and came back out of the net so quickly it was initially waved off. When officials reviewed the replay at the next stoppage of play, nearly three minutes later, they credited Tavares with the tally.

“Just a couple careless penalties that we didn’t need,” Schultz said. “It puts us behind the 8-ball with guys working too hard out there. Penalties at an unnecessary time.”

Frans Nielsen added a dagger of a short-handed goal, a perfectly placed shot over Grubauer’s shoulder with 3:01 remaining in regulation to make it 5-2.

That goal bothered Grubauer most. The 21-year-old was critical of his play, saying he “felt like I didn’t give the guys a chance to win.” But his Capitals teammates said Grubauer’s effort was the main reason the game was close entering the third period.

“It was real impressive to see him. He was calm out there and I think we owe him a win, actually,” Backstrom said. “There were too many goals there that we didn’t help him enough.”

Josh Bailey put the Islanders ahead 1-0 just past the midway point of the first period when he scored on an odd-man rush. Karl Alzner couldn’t flag down the puck in the neutral zone, sending Nielsen and Bailey up ice on a two-on-one. Nielsen fed the puck to Bailey on the left side for an easy shot as Grubauer slid across the goal mouth.

Washington evened the score with a lucky goal on a power play early in the second. Center Mathieu Perreault’s swipe at a loose puck in front caromed off the skate of Islanders defenseman Travis Hamonic and past goaltender Evgeni Nabokov (22 saves).

New York (11-11-3) continued to generate waves of chances and offensive zone time, though, fueled by Capitals turnovers. A giveaway by Alex Ovechkin in the neutral zone sprung the Islanders’ fourth line on another two-on-one and led to a goal by Casey Cizikas halfway through the second.

“We weren’t managing the puck very well. We weren’t executing many plays normally that we would have,” Alzner said. “I don’t know what it was but it didn’t seem to settle down for us the entire game so that didn’t help either. We weren’t very smart when we had the puck on our stick. “

Capitals note: Defenseman John Erskine left the game with what the team called an upper-body injury after skating just two shifts for a total of 1:18. It’s unknown whether he will be able to play Sunday against the New York Rangers at Verizon Center.