The Bruins, still unhappy about officiating decisions made in Thursday’s 2-1 loss to the Blues, know they must concentrate on their own performance on Sunday night if they want to bring the Stanley Cup Final back to TD Garden for a decisive Game 7.

BOSTON – One day later, it still wasn’t quite out of their system, but the Bruins’ disappointment with the manner in which officiating impacted them in Game 5 of the Stanley Cup Final on Thursday will have to be packed away by Game 6 on Sunday night (8, NBC, WBZ-FM 98.5).

That gives the Bruins, who trail the best-of-seven series 3-2, Saturday to work on a system they hope they’ll get to use more in Game 6, but which has stalled: their power play. It’s still the best among teams that qualified for the playoffs at 33.3 percent and has converted 31.6 percent of its chances in the Final against the Blues, but the power play is 0 for 5 over the last two games – both losses, each one effectively a one-goal decision.

Over Games 1-3 – after which Blues coach Craig Berube complained that his team was being unfairly penalized by officials – the Bruins were 6 for 14 (42.9 percent), including a perfect 4-for-4 (on four shots) in Game 3, which they won, 7-2.

“Our power play has kind of got stuck here a little bit,” B’s coach Bruce Cassidy said Friday at Warrior Arena, before he and the team departed for St. Louis. “We have to look at some adjustments.”

The Bruins were increasingly out of sync in Thursday’s 2-1 loss, when they went 0 for 3 with a manpower advantage. On a second-period opportunity, they lost the first faceoff and allowed the Blues to clear the puck, had three straight entries denied at the offensive blue line, then iced the puck on a set play.

Cassidy, whose second group had to play without defenseman Matt Grzelcyk (concussion) at one point position, switched net-front wingers Marcus Johansson (second unit to first) and Jake DeBrusk (first to second), but without results.

Whether it’s personnel or tactics, Cassidy suggested adjustments will be made for Game 6.

“I thought (the Blues) did a good job of influencing us to one side” when trying to enter the offensive zone, he said, “so we’ve got to be quicker identifying that. In-zone, they’ve been tight. (We’ve) either got to stretch them out, get some (passing) seams, or we’ve got to be less stubborn – get a net presence, and take the shot that’s available.

“A little bit is on us – ‘us,’ the staff – and it’s on the players to make the right decision, at the right moment in time.”

The Bruins haven’t hidden their opinion that a little is also on the officials to penalize the Blues when appropriate, which they don’t feel has been the case for the last two games. Tyler Bozak’s unpenalized slew foot, which took down B’s winger Noel Acciari in Thursday’s third period and helped keep the puck in the Bruins’ defensive zone long enough for David Perron to score the decisive goal, remained a hot topic Friday.

“My feelings haven’t changed,” said Cassidy, who included a high, unpenalized hit by Ivan Barbashev on B’s winger Joakim Nordstrom in the conversation (the NHL Department of Player Safety was reviewing the play on Friday). “I thought it was a missed call that impacted the game, unfortunately in a negative way for us.

“Sometimes, when you walk away and look at things, they executed one more play than us. That’s why they won – it wasn’t like we were horrible, and they were fantastic. I thought it was a well-played hockey game. That’s why you get frustrated (by) outside influences on the game.”

Cassidy knows that by Sunday night, the Bruins will either have to pack that frustration away or channel it in a more positive direction if they want to bring the series back to the Garden next Wednesday for a Game 7.

“We’ve played through certain situations; we’ve got to play through this one,” said the coach, citing officiating decisions in the first two playoff rounds that allowed goals by the Maple Leafs and Blue Jackets, respectively, to stand. “It’s not going to be brought up in the locker room.

“Our play has to define us on the ice, not a non-call, call, whatever. We have … high character. I suspect we’ll put it behind us, and off we’ll go.”