BOSTON — The gap between the Islanders and the best of the Eastern Conference remains demonstrably high — and the demonstration in question occurred on Thursday evening at TD Garden.

The Islanders left town with just as many questions as when they got here, along with the extra baggage of a 5-2 loss to the Bruins, their fourth in five games, to continue a tailspin that is in need of correction and fast.

“We are right now having trouble, for whatever reasons, sustaining a 60-minute game,” coach Lane Lambert said after his team’s record dropped to 5-4-3. “We’ve gotta find a way to do that.”

Charlie Coyle’s power-play goal 6:06 into the second period put Boston ahead 2-1 entering the third, but the results in the last 20 minutes were not so different with the Islanders playing from behind as they were when the Islanders held a lead.

It looked like the Islanders had some momentum after Simon Holmstrom scored shorthanded to tie the game just 10 seconds into a Boston power play.

But David Pastrnak quickly rectified that with a left-circle wrist shot that squirted through Ilya Sorokin’s pads and gave Boston a 3-2 lead 3:31 into the third.

“We’ve gotta find a way to kill the penalty off,” Lambert said. “We didn’t. … There’s too many times at these costly moments, we’re not up to the task at the moment.”

Coyle added another Boston goal at 9:26 of the period, getting to the backdoor to finish James van Riemsdyk’s feed as Samuel Bolduc was caught out.

More telling, though, was that for the second time in as many five-on-five goals against, the Islanders’ second line — with Pierre Engvall a healthy scratch — was on the ice.

At five-on-five, that was the story of the game.

With Anders Lee skating in Engvall’s place, the line with Brock Nelson and Kyle Palmieri did not have anywhere near the same speed or spark.

Engvall, whose speed is key to the line’s ability to transition, was badly missed and the Bruins’ line of Coyle, Trent Frederic and James van Riemsdyk dominated the game, as Coyle sealed his first career hat trick with a late empty-net goal.

“Well they’re minus-3, all of them,” Lambert said. “They know it. I thought they would be better.”

This was the first time this season that Lambert has attempted to send a message by benching a veteran.

But on Thursday, the message became that Engvall’s services are too important to waste in the press box.

He declined to go into detail on why Engvall was benched — though it figures that a costly defensive-zone turnover on Tuesday against Minnesota has something to do with it — saying only that the two of them “had a conversation,” the contents of which would be kept private.

“I think we had some O-zone shifts,” Nelson said, despite his line having been outshot 11-4. “I don’t think anything changes with Pierre out. Obviously I think it doesn’t matter who we have. We’re confident in our line to go out there and get it done.”

It doesn’t help, either, that Sorokin has lost five of his last six starts and was less-than-invincible on Thursday, stopping 30 of 34 shots.

“Sometimes, you go through waves during the season,” Jean-Gabriel Pageau said. “If you quit after the first [wave] of the season, you’re gonna be toast. We’ll just get back together.”

The slew of issues underpinning the Islanders’ game seemed a little bit more manageable in this game, which is to say they did not spend such a heinous amount of time in their own zone.

But taking steps forward is not the same as ameliorating the problems altogether.

Again on Thursday the Islanders had too many one-and-done chances and left potential grade-A looks on the table in failing to get shots off.

Still there are not enough pucks deep, not enough of a forecheck and as a result not enough sustained pressure.

Going 200 feet against the Islanders is not as hard as it should be, and that remains no small issue. And, most confounding, they are still losing steam in the third period.

No matter who is in the lineup, that cannot continue.