Through 40 minutes of play, the Bruins liked where they were against the Canadiens. Even with a tough call going against the Bruins — referees immediately motioned ‘no goal’ when Elias Lindholm appeared to score with 4.7 seconds left in the middle frame — the Bruins were still tied at 2-2 through two periods of play.

But in the third period, the wheels fell off the bus (and landed in the Charles for that matter), as the Bruins surrendered four goals on the way to a 6-2 loss.

The defeat was Boston’s fourth straight, and was more than enough to completely sour a five-game homestand entering the Christmas break, at 1-3-1.

“I think if you look at today, I still can’t believe that the game ended up 6-2,” Bruins head coach Marco Sturm said following the club’s loss. “Guys came ready to play today, they were very excited. Even the goals we gave up, it was a lot of individual mistakes and fatigue. Guys were just not mentally sharp.

“It seems like all week, we never had that confidence. Guys looked a little bit tired and that’s where you make a lot of those mistakes. We just couldn’t find our mojo.”

Speaking after the loss, the Bruins’ David Pastrnak did note that he believes that the team is going through “mental fatigue” at the moment.

But whether it was fatigue or just poor decision making, it was the Bruins’ continued penalty trouble that spelled danger for them in this one. Chasing the game after Montreal’s third goal, the Bruins took three minor penalties in the third period, and gave the Canadiens two separate five-on-three advantages.

“I think we’ve addressed it many many times,” Sturm said of Boston’s penalty woes. “It’s up to the players a little bit. Everybody has to ask themselves, ‘Do I really have to take that penalty?’ It can cost us games, points. It has to get better.”

Overall, the Boston penalty kill went 16-for-23 (69.6) during the five-game homestand, and allowed at least one power-play goal in all five games.

Both of Boston’s goals in this one came in the opening frame, beginning with a Marat Khusnutdinov net-front finish off a pretty feed from Mason Lohrei, while an Alex Steeves power-play marker gave the B’s a 2-1 lead through 20 minutes.

In goal, the Bruins’ Jeremy Swayman allowed six goals on 29 shots in the losing effort, while Montreal’s Jacob Fowler stopped 26 of 28 in the win.

Up front, the Bruins welcomed winger Viktor Arvidsson back to the lineup after what was a five-game absence with a lower-body injury. Arvidsson returned to his normal spot on a line with Casey Mittelstadt and Pavel Zacha, and finished with an assist and two shots on goal in 14:34 of time on ice.

On the backend, meanwhile, Vladislav Kolyachonok finally made his debut after being claimed off waivers back on Dec. 18. Playing on Boston’s third pair, Kolyachonok struggled to get much ice time in a special teams-heavy contest, but finished with three blocks, one shot on goal, and a penalty drawn in just 10:35.

The Bruins will now get a three-day break before making their way out to Buffalo for a Saturday night head-to-head with the Sabres.