Consistency. Every sports team preaches this. Establishing such a thing is easier said than done.

The Boston Bruins’ first 15 games are living proof.

Even in areas of improvement, like secondary scoring, the Bruins’ inconsistent performance against the Vancouver Canucks reared its ugly head Thursday night. Here is what we learned as the B’s fall to 8-5-2 following their ugly 8-5 loss at TD Garden.

A rough outing for Jaroslav Halak and Tuukka Rask

The days of Tim Thomas robbing the Sedin twins, Alex Burrows and the Canucks during the 2011 Stanley Cup Final are ancient. Jaroslav Halak and Tuukka Rask both wished they had it easy against the current core of Canucks — who have no leftovers from that championship series seven years ago.

Rask, who served as Thomas’ backup, is one of five members left from that Bruins 2011 Cup squad. He played the same role to start Thursday’s regular season tilt with a younger Canucks squad but put in a relief appearance after Bruce Cassidy pulled Jaroslav Halak — who allowed five goals on 19 shots — with under five minutes left in the second period.

The 2014 Vezina winner didn’t fare much better allowing three goals on 14 shots in 25:07 of ice time. Two of those three goals — one soft goal allowed from Erik Gudbrandson and an ugly mishap that led to Bo Horvat’s second of the night — couldn’t have come at a worse time as the Bruins pushed to keep things close during a wild middle stanza.

“Neither guy was on their game, so that’s a problem,” Cassidy said about Halak and Rask. “It’s going to happen. Defense, I think it was as much turnovers worked against us once we started to get behind.”

Certainly, the Bruins D didn’t help things as the Canucks got in front on transition and created traffic in front of both goalies. But neither could make the timely save when needed.

Rask and Halak combined for eight goals allowed in 60 minutes, the same number of goals that Thomas gave up in seven games against the same — yet different — Cancuks team seven years ago. Certainly not duck boat worthy.

“We just, obviously, we have to be better,” Halak said. “We have to be better as a group and that’s the bottom line.”

“I was just trying to keep it under 10 – that’s what I was worried about,” a sarcastic Rask added. “But yeah, you know, like I said, a loss is a loss, it doesn’t really matter at the end of the day.”

Tide swings with missed opportunities

A team that scores five goals on any given night often comes away with two points. Not on this performance.

Despite the plethora of goals going the other way, the Bruins converted several chances and had ample shots to cash in on a few more. But David Krejci hit the crossbar following Jake DeBrusk’s second of the night late in the second period — leading to Gudbranson’s first of the season — and a 5-on-3 power play opportunity in the third went haywire after Horvat picked Rask’s pocket to give the Canucks a three-goal cushion.

“Yeah, you can obviously look back at certain moments of the game. I think going into the third period we wanted to get one quick with that 5-on-3,” DeBrusk said. “If we score on that we get some momentum, but obviously it wasn’t in the cards tonight. It just didn’t work out the way we wanted to.”

Secondary scoring finally gets going

Silver linings are the last thing on any team’s mind, but there’s one good talking point worthy of discussion from this strange night on Causeway Street.

The Bruins had struggled to generate any goal scoring outside of their potent top line of Brad Marchand, Patrice Bergeron and David Pastrnak. The trio still had their chances and factored into two of Boston’s five goals, but they had some good company during the team’s nightmarish performance.

DeBrusk, showing shades from his impressive playoff debut against the Maple Leafs last spring, played his best game of the season en route to a three-point night. Defenseman Matt Grzelcyk blasted a slap shot past Jacob Markstrom for his first of the season at 13:41 of the opening period. Forward Danton Heinen joined Grzelcyk in that first goal department with a garbage goal at 13:38 of the third.

Even Monday’s effort against Dallas from the second, third and fourth lines drew praise from Cassidy. That effort finally paid dividends three nights later.

No time for consolations, though. The Bruins know they’ll need more secondary scoring and better efforts from their D and goaltending in a rare home back-to-back against the high-octane Toronto Maple Leafs and defending Western Conference champion Vegas Golden Knights this weekend.

“Well, two good teams. Toronto obviously a division rival that we should have no problem getting up for, especially after tonight, but no matter how it went tonight we had a good series with them last year, so I imagine there will be – there better be some energy in that game. The next night, back-to-back, Vegas will have – they’ve had a busy week like us. I think it’s the exact same schedule for them, so that’s one of those games that mentally we need to make sure we’re on the ball and don’t beat ourselves until we find our legs,” Cassidy said.

“But again, it’s one step at a time, and for us that’s tomorrow, have a good practice, fix a few things, then be sharp against Toronto.”