Blackhawks goaltender Robin Lehner blocks a scoring chance by Islanders...

Blackhawks goaltender Robin Lehner blocks a scoring chance by Islanders left wing Michael Dal Colle during the second period on the way to a 5-2 victory over the Islanders on Friday, Dec. 27, 2019, in Chicago.   Credit: AP/Matt Marton

CHICAGO — Not having Robin Lehner was far from the Islanders’ only problem on Friday night.

More problematic was the shaky defensive work from the net on out and an alarming drop-off in the Isles’ work ethic.

“You get as much out as you put in,” coach Barry Trotz said. “We haven’t put enough in. We’ve got a little cheat in our game. It’s hard to be hungry when you’re full, and we’re playing a little full.”

The Islanders, forced to fly on the day of the game because of collective-bargaining agreement guidelines coming out of the NHL’s three-day holiday break, were outskated in a 5-2 loss to the Blackhawks and their former fan-favorite goalie.

“There’s a few guys there that are not allowed to score,” Lehner, who made 38 saves, told a larger-than-usual group of media members surrounding his dressing room stall after the morning skate.

Ross Johnston beat Lehner for a 1-0 lead at 5:50 of the first period, but the Blackhawks (16-17-6) scored the next five goals.

The Islanders (23-10-3), who opened a three-game road trip, have allowed 24 goals in a 1-3-1 skid and are 0-5 against the bottom five teams in the Western Conference: the Blackhawks, Kings, Sharks and Ducks.

Thomas Greiss, who shared the Jennings Trophy with Lehner last season as the Islanders allowed the fewest goals in the NHL, was pulled after allowing three goals on 11 shots in 11:56 as he made consecutive starts for the first time this season.

“It’s just mistakes in our own end that are killing us,” said Casey Cizikas, who closed the scoring with a shorthanded goal, his third of the season and the Islanders’ sixth, at 16:32 of the third period. “We’ve got to find a way to be heavy and play a full game. We need everybody. We’re getting away from that. We’re not playing Islanders hockey and it’s getting ugly.”

Said Anders Lee, “We’re a team that works and we’ve got to get back to that.”

Lehner was a Vezina Trophy finalist as he went 25-13-5 with a 2.13 goals-against average and a .930 save percentage. He detailed his struggles with addiction and mental-health issues on the first day of training camp and went on to win the Masterton Trophy for perseverance.

He concentrated on re-signing with the Islanders, but negotiations were unsuccessful and he signed a one-year, $5 million deal with the Blackhawks.

“It’s special playing the Islanders,” said Lehner, who shared a postgame hug with Trotz outside the Islanders’ room before chatting with some of his ex-teammates. “They mean the world to me. It’s a heck of an organization. They supported me. But it’s nice to get the win.”

“He made some nice saves,” Mathew Barzal said. “But we didn’t really test him.”

Defenseman Dennis Gilbert, with his first NHL goal, tied the score at 1 on a backhander from the slot at 7:34 of the first period. Dominik Kubalik beat Greiss from the slot to give the Blackhawks a 2-1 lead at 9:46 and Alex DeBrincat ended Greiss’ outing by beating him over the glove for a 3-1 lead at 11:56.

Lehner started off the second period sharp, stopping Jordan Eberle’s backhander from the slot and gloving defenseman Nick Leddy’s wrist shot from the left faceoff circle in the first two minutes.

The Blackhawks then put the game out of reach. Jonathan Toews stripped defenseman Ryan Pulock of the puck and scored on the rush 50 seconds into the third period before Matthew Highmore made it 5-1 at 3:03 with a feed that deflected in off Pulock past Semyon Varlamov (17 saves).

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