BUFFALO, N.Y. -- With starting goaltender Jaroslav Halak sidelined, Thomas Greiss continues to impress between the pipes for the New York Islanders.

Greiss made 42 saves Thursday night while leading the Islanders to a 2-1 victory over the Buffalo Sabres. Greiss withstood an early barrage and was there again late to preserve the win.

"Thank God for him," Islanders coach Jack Capuano said. "Our goalie came up huge for us."

It was Greiss' third start in a row for the Islanders (21-12-5). Halak is out with an upper-body injury but is expected to return soon.

Center Frans Nielsen and left winger Nikolay Kulemin scored for New York.

Greiss' best moments came in the first period following a lackluster start by his team.

The Sabres were all over the Islanders through the first 20 minutes, outshooting Buffalo 17-5. Through the first 14:42 of the period, Buffalo held a 16-1 advantage in shots. The Sabres also had three power plays -- including a five-on-three advantage -- but Greiss was there to thwart their efforts.

"It's always nice when a lot of pucks get to you quick and early in the game," Greiss said. "Had a lot of shots, but the team worked well to clear rebounds and helped me out there as much as they could."

The Islanders were much improved after the first period.

Nielsen opened the scoring midway through the second period against the flow of play by pouncing on a loose puck in the Sabres' crease.

After a shot from Islanders center John Tavares was initially stopped, Ullmark appeared to have control, but the puck was actually beside him. The Sabres challenged the goal, but it was confirmed. Nielsen's goal came at 9:35.

"I heard the crowd, that sound where something is loose in there," Nielsen said. "I started looking and suddenly it was sitting there right on the goal line."

Kulemin doubled the lead 1:07 into the third period. Right winger Kyle Okposo found Kulemin with a backhand pass from behind the Sabres' net, and Kulemin struck for short range for his fifth goal of the season. It was his third consecutive game with a goal.

Center Ryan O'Reilly scored for the Sabres (15-19-4). Buffalo goalie Linus Ullmark made 31 saves.

"Did a lot of things you want to do in the game," Sabres coach Dan Bylsma said. "In particular, the power plays in the first period, the shot volume was there, had good looks, had net fronts, had pucks lying in the crease. We had a ton of opportunities tonight, we just weren't able to put them home."

O'Reilly got the Sabres on the board with a power-play goal with 8:42 remaining in the third period following a strong setup by Jack Eichel. The rookie center fed a hard cross-ice pass to O'Reilly in the left circle, and O'Reilly made no mistake, putting home his 15th goal of the season.

It was Buffalo's first power-play goal in 24 attempts over a span of eight games.

"We had a chance to take the game and run away with it, and we didn't," O'Reilly said.

Islanders defenseman Johnny Boychuk left the game in the third period after going headfirst into the boards. Boychuk went to put a hit on Eichel and missed before crashing into the end boards. Capuano said after the game that Boychuk avoided a serious injury on the play.

"Fortunately for us that Johnny Boychuk came off on his own power and he's day-to-day, upper body," Capuano said. "But it could've been a lot worse, his neck or his head, and just thankfully he's OK."

NOTES: Sabres C Tyler Ennis and D Carlo Colaiacovo were scratched. Ennis sustained an upper-body injury in the second period of Buffalo's 5-2 loss to Washington on Wednesday. ... Islanders RW Steve Bernier, LW Eric Boulton and D Brian Strait were scratched. ... Islanders G Jaroslav Halak is closer to returning from an upper-body injury. He rejoined the team in Buffalo for practice but remains on injured reserve. ... This was the second of three meetings between the Sabres and Islanders this season. The third and final game takes place April 9 in New York.