Jack Capuano was adamant that his team has not quit. And yes, the Islanders did rally from a two-goal deficit in the second period to pull even with the Senators on Sunday night.

But the two areas of their game that have slipped in a huge way during the past week bit them again: their penalty kill and their third periods.

Mark Stone’s power-play goal untied it late in the second period and the Isles went without a third-period goal for the fifth straight game, undone in a 6-2 defeat at Barclays Center that was their fifth straight loss (0-4-1). That just about fully negates the 5-0-1 run they went on to pull out of the Eastern Conference basement.

“We have to play better,” Johnny Boychuk said. “We have to play with some desperation. Every point at this point matters. We’ve put ourselves in this position and we have to play every game like it’s very important.”

After Anthony Beauvillier tied it at 2-2 at 14:07 of the second period, Stone beat J-F Berube at 15:03. It was the eighth power-play goal in 15 opportunities the Islanders have allowed in the five losses.

Berube, getting his second straight start after not playing the first two months of the season, was beaten by three oddball goals of the four he gave up (there were two empty-netters). Stone’s shot banked off Travis Hamonic’s skate and in. Even so, the Isles were not good enough.

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“It’s three of our guys to their two down low,” Capuano said. “We have to find a way to tie up a stick or make a play.”

Capuano said his team has not flagged in its effort or “battle level,” as the coaches like to say these days. Now at 11-14-6 and facing a near-impossible climb to get back in the playoff hunt, the beleaguered coach defended his players, as he’s always done.

“When teams lose it, they start quitting, there’s no emotion,” he said. “That hasn’t happened with this group.”

But what has happened to them? The Islanders were not world-beaters in 2015-16, but one of their signatures was an ability to outwork opponents to the final horn. They were plus-15 (87-72) in third-period goals a season ago and plus-4 (91-87) a season before that.

Now they’re minus-10 and dropping, having allowed 11 third-period goals (plus one in OT) and having scored none of their own in the five-game slide.

“We’re not coming out in the third periods the way we should be,” said Anders Lee, who scored his team-high 10th goal. “Good teams grind out a point or two. It seems like everyone’s doing that to us.”

The Islanders briefly thought they had tied the score in the third period when John Tavares danced through three Senators and snapped a shot that beat Mike Condon. Play continued, but referee Graham Skilliter emphatically ruled goal. He skated to the penalty box to consult with Toronto while Tavares went to the bench, stone-faced. He knew the truth.

“I didn’t have a positive feeling,” he said of the shot that clearly hit the crossbar and stayed out. “I saw it pretty good the whole way.”

Just 75 seconds later, Derick Brassard picked up a convenient bounce off the end wall from Stone’s shot and stuffed it behind Berube to make it 4-2.

“You can talk about it any way you want,” Tavares said glumly. “We lost the game. That’s all that matters.”