Brock Nelson is sure glad the New York Islanders acquired Jean-Gabriel Pageau at the NHL trade deadline in February.

Islanders coach Barry Trotz wasn’t sure what the score would have been if not for backup goalie Thomas Greiss' performance in his first start in nearly six months.

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Pageau scored the go-ahead goal 7:18 into the third period, and Greiss stopped 36 shots — including a whopping 16 in the second period — to push the Islanders within a win of advancing to the Eastern Conference finals following a 3-2 victory over the Philadelphia Flyers on Sunday night.

Nelson scored twice, both set up by Josh Bailey, and the Islanders gained a 3-1 lead of the best-of-seven second-round series with Game 5 set for Tuesday night.

New York is seeking to make its first conference finals appearance since 1993, when the Islanders lost to Montreal to six games.

None of that success would’ve have been possible if not for Greiss, who got the start in place of Semyon Varlamov with the teams playing on back-to-back nights following New York’s 3-1 win on Saturday.

“Tonight there’s no question that `Greisser’ was a big part of the solution in what my opinion was the worst game we’ve played to this point in the playoffs,” Trotz said of the goalie making his first start since a 3-2 overtime loss to Carolina on March 7.

He was referring to a second period which ended with the score tied at 1 despite the Islanders being outshot 17-3 and going nearly 11 minutes without a shot on net.

Then Trotz credited Pageau for scoring his seventh goal of the playoffs on a breakaway.

“After that second period, we needed something to sort of push them back, and that did it,” Trotz said.

Sean Couturier and Ivan Provorov scored for the East’s top-seeded Flyers, who lost consecutive games for the first time since an 0-3-1 skid from Dec. 31-Jan. 7. The Flyers also went with backup Brian Elliott, who stopped 30 shots in place of Carter Hart.

Though Provorov scored with 64 seconds left and the Flyers net empty, Philadelphia managed just one more shot on goal, as the Islanders muddled up the puck in their own corner for much of the final 20 seconds.

Though the Flyers finished out-shooting the Islanders 38-33, a lack of finish and defensive breakdowns cost them.

“We busted a gut out there. We worked our (rears) off from the start of the game till the end,” coach Alain Vigneault said. “In the third period, we made a couple of mistakes and they made us pay for it. We had some looks and just couldn’t find a way to put more than we did past their goaltender.”

The Flyers were caught flat-footed in allowing Pageau to sneak in behind them.

Defenseman Scott Mayfield jumped into the rush to set up the go-ahead goal. Accepting a pass from Devon Toews, Mayfield fed Pageau for a breakaway. Pageau drove down the middle and sneaked a shot through Elliott’s legs, with the puck having enough spin on it to dribble over the goal line.

This is the type of clutch production the Islanders were seeking in giving up three draft picks, including a condition first-rounder this year, to acquire the 27-year-old play-maker from Ottawa on Feb. 24.

“The payoff for him is obviously paying off right away,” Nelson said of Pageau. “He’s a guy you can lean on in any situation. He kind of does it all. He’s a guy you appreciate and love to have him on your team.”

Nelson made it 3-1 by scoring four-minutes later by completing a give-and-go with Bailey on a two-on-one break. Driving the Flyers zone up the right wing, Nelson fed Bailey, who returned the pass allowing Nelson to tap the puck in the open left side.

Greiss, meantime, was sharp throughout. Midway through the first period, he kicked out his right pad to foil Joel Farabee’s redirection. With two minutes left in the second period, Greiss shot up his glove to stop Kevin Hayes snap shot off a faceoff.

The Flyers’ offense has essentially dried up, with captain Claude Giroux having yet to score this postseason.

After combining for 11 goals in sweeping the top-seeded playoff-opening round-robin three-game series, Philadelphia has managed just 18 in its past 10 outings.

“We have to find ways to score a goal,” said Giroux, who played a big role in setting up Couturier’s goal, even though it didn’t lead to an assist. “Obviously, a little frustrated. Not too sure what to say. I know I can play better.”

NOTES: Flyers LW Oskar Lindblom took part in the pregame warmup before being scratched. The 24-year-old has rejoined the team and hasn’t played since Dec. 7 when he was diagnosed with a rare form of bone cancer. ... Islanders have three players -- Mathew Barzal, Josh Bailey and Anthony Beauvillier -- with at least 10 points in one postseason for the first time since 1993. ... With a win in Game 3, Varlamov (9-2) joined Billy Smith (1980 and ’81) to win at least nine of 11 decisions in one playoff.

UP NEXT

Game 5 is Tuesday at 7 p.m. EDT.