If the Islanders are to make a run at this for real, then Thursday must be only a mirage — the kind of performance they get over and quickly put behind them.

This was not their best and not even close to it, as the Islanders blew a two-goal lead to lose at home to the lowly Canucks, 6-5, snapping a four-game winning streak after a game-winning goal from Anthony Beauvillier in his return to UBS Arena. It is a survivable calamity, but a blown chance at two points, and an addition to a long list of games the Islanders have lost against subpar competition. With their next two games looking winnable — at Montreal and home to Ottawa — they can afford this loss in solitude. But they cannot afford another spiral.

“From the start, I think they outplayed us,” Kyle Palmieri said. “And there was no excuse for it. The whole game’s not something we want to put out there.”

On Thursday, the nature of the errors changed as the game went on, but it was the same trend. The Islanders did not have enough of the puck and did not sustain enough pressure against a vulnerable goalie to win a game they had in their grasp. That was a direct consequence of failures to break the puck out.

“I saw a lot of mistakes being made, a lot of individual turnovers,” a seething Lane Lambert said. “Twenty-three giveaways, you’re not gonna win a hockey game doing that. There was plays to be made and we didn’t make them tonight.”

And in truth, the official count of 23 giveaways probably did the Islanders a favor. Even so, they held a 4-2 lead before Nils Aman cut it to one with just under four minutes to go in the second period. In the third, Vancouver went on to steal the game.

Adam Pelech caught Vasily Podkolzin with a high stick, and Elias Pettersson took advantage to blast home a shot from just below the blue line and tie the game at four under five minutes into the third.

Then, after another Islanders failure to break out of the zone, it was Pettersson again who put away a shot on Ilya Sorokin’s glove side to make it 5-4, 8:28 into the third.

Lead blown.

The Islanders did themselves no favors for the rest of the game, devolving into sloppy and disorganized hockey, failing to make a push until a Vancouver penalty gave them a chance at six-on-four. The intended results did not come, and neither did the comeback.

Of all people, it was Beauvillier who tipped a puck past Sorokin with 3:02 to go, sealing Vancouver’s win. Noah Dobson pulled the Islanders back within one at six-on-four, but it was too little, too late as a scrambling attempt to tie it fell short. Sorokin let in an uncharacteristic six goals on 34 shots.

“When you’re up 4-2 in a hockey game late in the second period and you don’t win the hockey game, you feel like you’ve let something slip away,” Lambert said. “And there’s a certain amount of frustration with that.”

Even when the Islanders held a two-goal lead, though, this was far from their best game — a fact Palmieri openly acknowledged afterwards. Both of their first two goals, which kept the game tied in an opening period dominated by Vancouver, came on rush chances that got past Collin Delia. The next came right after a power play expired.

And, though Bo Horvat scored against his old team to make it 4-2 on a flourishing finish off Mathew Barzal’s feed, even the top line that looked so good in its first two games together didn’t see enough of the puck on Thursday.

“You watch the last couple games before tonight, you see five guys in the picture when we’re breaking pucks out,” Dobson said. “We’re helping each other out. Just finding at times we’ve been getting disconnected, like tonight. Too much gap between our five guys. We gotta help each other out, especially when teams are forechecking hard.”

Lambert put it more simply.

“It’s unacceptable at this point,” he said.

Indeed, he would be correct.