Dansk left the game following the goal. He made 17 saves. Vegas is already without goalies Marc-Andre Fleury and Malcolm Subban because of injuries.
"The medical guys will look at him tonight or tomorrow, so I really don't have an answer for you right now," Gallant said of Dansk.
Barzal then gave the Islanders their first lead with a power-play goal at 17:44 when he one-timed Anders Lee's feed from the left circle to make it 3-2.
"I think the last two games we've had things going in the right direction," Bailey said. "The power play, obviously we had a bit of a slip-up there, but all in all it's been moving in the right direction."
Clutterbuck gave New York a 4-2 lead when he beat Lagace with a wrist shot from the right circle at 4:44 of the third period.
Kulemin brought Vegas within 5-2 when he scored his first of the season on a breakaway at 8:26 before Tavares scored his second of the game at 12:38 to make it 6-2.
Miller made it 6-3 with a power-play goal at 14:41.
Lagace allowed four goals on 11 shots in his NHL debut.
"Not the start I obviously wanted for my first NHL [appearance], but I'll watch the video and see what I can do better and then right back at it [Tuesday]," Lagace said.