In a year of transition, there’s no denying that this year’s Boston Bruins squad was going to see a lot of ups and downs.

By the end of last week, things were looking up for the Black and Gold after they swept the Florida trip to extend their winning streak to four games. They were getting timely goals, solid defense and great efforts in victories over the Islanders, Coyotes, Panthers and Lightning.

Six days later, things are looking downward again. In a week that already saw losses to the Stars and Capitals, the difficult four-game stretch continued with the league-leading Montreal Canadiens on Saturday night.

In this one, the B’s had their hated rivals right where they wanted them. They were physically engaged and dictated the tempo to a 2-1 second period lead against a Habs team that looked a little off in their passing.

Still, that wasn’t good enough. The Bruins couldn’t take advantage of a few breaks, including a successful coaches challenge in the third period, and the Habs caught them on their heels with three unanswered goals en route to a 4-2 triumph at the Bell Centre. Here are a few things we learned from Boston’s third straight loss.

Bruins lack discipline, composure in third

The last thing that the league’s worst penalty kill needed was one of their own committing an untimely penalty.

Following Jimmy Hayes and Brad Marchand’s second period minors that led to the Capitals extending their lead to two goals, the Bruins shot themselves in the foot again when David Krejci — who had an uncharacteristic three trips to the penalty box — took an unnecessary cross-check on Tomas Plekanec with 2:03 left in regulation. Less than a minute later, David Desharnais gave the Canadiens their second power play goal and first lead of the night. Twenty seconds following Desharnais’ go-ahead goal, Max Pacioretty delivered the dagger with the empty netter.

“That’s two nights in a row where we blew games, or we lost games because of a lack of discipline,” a disappointed coach Claude Julien told NESN’s Jack Edwards and Andy Brickley afterwards. “The ill-advised penalties [against Washington] that put us down 3-1 and tonight you have a tie game and you take a penalty with 2:03 left. It’s disappointing because it’s coming from our leaders and our leaders have to lead the way, but they’re not leading the way properly right now.”

As one of the assistant captains, Krejci, who has not recorded a point in four straight games, would not deflect responsibility for his untimely penalty.

“It’s disappointing, especially because [our] guys did a very good job and played hard,” Krejci told reporters. “And then you do something stupid, and I let everyone down. So…not a very good feeling.”

Certainly the Bruins’ effort was there. But they shot themselves in the foot with a lack of discipline, not capitalizing on golden opportunities (like Torey Krug fanning on a wide open net during a power play opportunity) and Jonas Gustavsson’s sloppy rebound that led to Lars Eller’s tying goal. Against the best team in the league, they have to find a way to win, and that is the major difference between the two rivals.

Frank Vatrano scores in NHL Debut

One of the bright spots for the Bruins on Saturday, Frank Vatrano earned the most of his first chance.

After leading the AHL in scoring during his time in Providence, the former UMass Minuteman picked up right where he left off in his first career NHL action. Skating on a line with Krejci and Loui Eriksson, Vatrano fired three shots on net, earned 14:20 of ice time (including some power play action) and scored his first career goal at 8:51 of the second to give his team a 2-1 lead.

With David Pastrnak still nursing a foot injury, Vatrano is giving Julien another scoring option. Even after his first career game, I would be surprised if the East Longmeadow-born forward was sent back down to Providence anytime soon.