ELMONT, N.Y. — Everyone with the New York Islanders knew the stakes. Perhaps they weren’t aware of the exact math — how a regulation loss to the Pittsburgh Penguins would drop their playoff odds from 63.3 percent to below 50 percent, according to colleague Dom Luszczyszyn’s model — but the importance of this game was a given. A division opponent competing for a playoff spot. Necessary points on the line. A home crowd engaged.

And the Islanders responded with about as bad a game as possible.

“We lost a big game tonight,” captain Anders Lee said after an 8-3 thrashing in which his team blew a 3-1 lead.

Added coach Patrick Roy: “I guess we just had a bad one in the system after that second (Penguins) goal.”

That’s costly this time of year. The Islanders (89 points in 75 games) fell behind Pittsburgh (90 points in 74 games) in the Metro Division standings, and the Penguins have a game in hand. By point percentage, the Columbus Blue Jackets are ahead of the Islanders (.595 to .593), though New York has a 1-point edge in the Metro standings.

Roy’s team has enjoyed a pleasantly surprising season, led by Ilya Sorokin’s brilliance in net and No. 1 pick Matthew Schaefer’s emergence as an 18-year-old star. But if the Islanders have many more games like Monday’s, the charmed year will end without the playoffs. They have a 49.3 percent chance, per Luszczyszyn’s model. They could make the playoffs as a wild-card team should Columbus hop them, but other clubs — the Ottawa Senators, Detroit Red Wings and now red-hot Philadelphia Flyers — are all in the mix for one of those two spots, currently occupied by the Boston Bruins and Blue Jackets.

So, given the crowded Eastern standings, the Islanders played the wrong way Monday against the wrong opponent.

“This is like a playoff game,” Lee said afterward. “Their momentum swings were just longer than ours.”

Helped by a strong first period from Sorokin, who fended off a dangerous Erik Karlsson shot in the opening minutes, the Islanders withstood a Penguins push in the first period. They easily could’ve been trailing — Pittsburgh had 65.26 percent of the five-on-five expected goal share that period, per Natural Stat Trick — but went to the dressing room tied at 0. Then Roy thought his team seized the momentum, jumping out to a 3-1 lead. But a short-handed goal from Rickard Rakell, who got behind Adam Boqvist and Emil Heineman, began a Pittsburgh siege.

The Islanders defense fell apart in front of Sorokin. Roy called Monday “one of our worst games defensively in a while.” The numbers back that up. Natural Stat Trick credited the Penguins with a 13-1 lead in five-on-five high-danger chances in the second period, and Roy said his team gave up 11 chances, presumably using the Islanders’ data.

“We can’t give that many chances and think we’re going to win hockey games,” he said.

The Penguins’ fourth line found a loose puck for a goal against Mathew Barzal’s line, which had only 35.99 percent of the five-on-five expected goal share on the night, per Natural Stat Trick. Anthony Mantha sneaked past the Ondřej Palát–Jean-Gabriel Pageau–Simon Holmström line for Pittsburgh’s go-ahead and insurance goals. There weren’t many players who could feel good about their performance; the back half of the game, starting with the second-period implosion, was “difficult for every one of us,” Roy said.

Sorokin, arguably the Vezina Trophy front-runner, has been brilliant this season. He can stave off bad stretches of play in front of him, such as when the Islanders allowed a couple of dangerous chances in the first, but if those chances come in relentless waves, as they did in the second, there’s nothing to be done. New York and Pittsburgh combined for eight goals in the period, tied for the most in a single frame in any game this season. Unfortunately for the Islanders, they surrendered five of those goals — as well as anything positive they built with their early-period scoring.

Midway through the third — after the Islanders defense gave up another odd-man rush and Rakell scored his second goal of the game — Roy made a mercy pull, subbing in backup goalie David Rittich. Sorokin perhaps wasn’t his sharpest, but the blame lay on those in front of him.

“We let (Sorokin) down,” forward Kyle MacLean said. “He’s been so good for us, and we can’t play like that in front of him. He deserves better.”

“Every goal, you can do something more, but (they were) good goals,” Sorokin said. “Every goal is hard to stop.”

Roy said the game hurt in part because “we’ve been doing so well lately.” The loss broke a three-game streak of the Islanders winning the five-on-five expected goal battle, per Natural Stat Trick. With a road game against a strong Buffalo Sabres team Tuesday, New York quickly has to regroup. It’s the type of game the team will need to win down the stretch: The Islanders have the hardest remaining schedule, per Tankathon, and they’ll need to be much better than they were against Pittsburgh.

“I give (the Penguins) credit for going at the net and being at the net,” Roy said. “They did a really nice job winning those battles on those loose pucks. These are things we need to do better as a team if we want to win those big games.”