A six-goal first period Thursday night at TD Garden put the Boston Bruins behind early in the eventual 6-5 loss to the Pittsburgh Penguins.

The Boston Bruins’ incomplete comeback against the Pittsburgh Penguins Thursday night at TD Garden saw them fall 6-5 after a sour first period.

Despite a game-tying, short-handed goal from Brad Marchand to make it 5-5 at 3:08 of the third, Boston’s lackadaisical start cost them in the end.

“It’s just we’re not making sound decisions, we’re forcing stuff when we don’t need to,” Bruins coach Jim Montgomery said Thursday.

“Unfortunately, our game management I think cost us the game. That being said, you’ve gotta love, as a coach, you love the way your team never stops fighting.”

It was a first-period frenzy with the game’s first five goals all scored within the opening 7:21 of play, and six total by the end of the 20 minutes.

While Marchand and David Pastrnak both scored for the Bruins in the first, the Penguins went into the middle frame with a 4-2 lead thanks to goals from Drew O’Connor, Ryan Graves, Jake Guentzel and Lars Eller.

“When they were making plays off the rush, they were high-danger plays. We’ve got to limit those and manage our game a little bit better and play a little bit better defensively for sure,” Brandon Carlo said Thursday.

Boston didn’t look good anywhere on the ice, but especially collapsed in the defensive zone and left Jeremy Swayman – who ended the night with 29 saves – out to dry. Spotty coverage, weak puck support and an overall lethargic effort to transition properly dug the B’s in a deep hole from the beginning.

“A couple bounces there early that we’d like to have back, but missing coverage against a team like that can’t happen so we need to be better there,” Marchand said.

Montgomery kept Swayman in for the entirety of the game despite the six goals against, and said he wanted him to “fight through it.”

“Sometimes you give them the opportunity to fight through it, and when it’s a game where it’s back-and-forth like that, I thought that was the opportunity,” Montgomery said.

Penguins forward Jeff Carter extended his group’s advantage at 6:38 of the second period, first deflecting and then knocking in the rebound of Kris Letang’s shot from the point for a 5-2 lift.

The Bruins brought themselves within one heading into the third with goals from Morgan Geekie and Brandon Carlo. A right-side wrister from Geekie at 9:51 made it 5-3 before Carlo’s blast from the top of the zone cut the deficit to 5-4.

“We definitely had our opportunities,” Geekie said Thursday. “Like I said, this is something we can build off of for sure and we’ll put what we need to in the rear-view mirror and keep building off it.”

In the final stanza, Marchand muscled his way around Kris Letang on the short-handed break, earned inside ice as he closed in on the net and roofed the puck over goaltender Alex Nedeljkovic’s left shoulder to tie things 5-5 at 3:08.

“His bulldog strength, how he gets under a guy, frees himself up, gets his hands loose. It really energized the bench,” Montgomery said. “It’s one of those second and third effort plays [that] motivates and inspires his teammates.”

However, the Penguins then struck on Charlie McAvoy’s second penalty of the period – a hooking call at 11:12 – and cashed in six seconds into the man advantage to secure the final 6-5 score with a Sidney Crosby wrister from above the left circle.

The Bruins will be back in action Saturday night at TD Garden where they’ll welcome the Tampa Bay Lightning for a 7 p.m. puck drop. 

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