By Michael Hurley, CBS Boston

BOSTON (CBS) — Tuukka Rask has been arguably the Bruins’ best player during their postseason run. On Monday night in Game 5, he was not. So Bruce Cassidy gave him a seat on the bench for the third period.

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After Rask allowed four goals on 16 shots, he came out for the third period with a towel on his shoulder and a seat at the end of the Bruins’ bench.

Rask was replaced by rookie Jeremy Swayman in the third period. Swayman came out of seemingly nowhere to shine for Boston this year when both Rask and Jaroslav Halak were out of the lineup for an extended period of time. Swayman went 7-3-0 with a .945 save percentage, 1.50 goals-against average and two shutouts in his regular-season work, earning a spot on the playoff roster over the veteran Halak.

Unfortunately for Swayman, his playoff debut didn’t get off to the smoothest start.

He began his outing by turning aside a Jordan Eberle bid just a minute into the third period, followed by another stop on Mat Barzal 24 seconds later. But the Islanders maintained possession in the Boston zone, and Brock Nelson was left all alone in the slot. Anthony Beauvillier found the open man, and Nelson ripped a shot past Swayman to give the Islanders a 5-2 lead.

Rask had allowed four goals, three of which came on the power play. He was beaten cleanly by a Barzal shot on the first goal, but the second goal bounced off three skates before ending up on the tape of Kyle Palmieri on the doorstep. Josh Bailey’s goal — New York’s third of the night — was similar to Nelson’s, as it came on a shot from a highly dangerous spot on the ice. And the final goal allowed by Rask came when the Bruins were essentially playing two men down after Jarred Tinordi broke a stick on the penalty kill, ultimately leading to a passing lane opening for Barzal to find Eberle for a picture-perfect power-play goal.

Goaltending wasn’t necessarily the Bruins’ problem on Monday night. But it also wasn’t an asset. So Cassidy made a change.

The goal allowed by Swayman ended up being the game-winner, as the Bruins fought back to make it a 5-4 game but couldn’t get the equalizer.

After the loss — which now has the Bruins facing elimination on Wednesday night in New York — Cassidy said that Rask needed some “maintenance” and wasn’t “100 percent,” which led to the change.

“There was some maintenance that needed to be done. He wasn’t 100 percent. So I made a decision,” Cassidy said. “And that’s the call we had to make between periods. I’ll just say he wasn’t himself 100 percent. Certainly could have went back in [for the third period], but we made a decision not to put him back in.”

After allowing the goal, Swayman didn’t face any more shots, finishing his debut with two saves on three shots.

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It likely won’t mean anything with regard to a starting goaltender for Game 6 at Nassau Coliseum on Wednesday night, as Cassidy said he “assumes” that Rask will be ready to go. But until that game begins, it’s certainly something that seems slightly unsettled.