BOSTON • The NHL’S best team added insult to injury for the defending Stanley Cup champions.

Literally.

It was a forgettable Saturday night at TD Garden with the Avalanche falling 5-1 to the redhot Bruins. Colorado’s lack of depth amid ongoing health woes loomed large. Top-line winger Artturi Lehkonen left the game with an upper-body injury in the first period and did not return.

How bad has it gotten for the bruised and battered Avalanche? They’re now missing four of their top-six forwards and two of their top-four defensemen.

“We have to be mentally tough. That’s the biggest thing,” coach Jared Bednar said. “We still have the ability to win hockey games, although the margin for error is shrinking. You feel like at times you have to be perfect in order to win. We certainly weren’t that tonight.”

Colorado, in the midst of a four-game road trip, faced their greatest test of the young season against a Boston team with an Nhl-leading record (20-30) while undefeated (14-0) at home. Bednar said after morning skate: “It’s an easy game to get up for. You’re playing one of the top teams in the league.”

But the Avalanche — dropping to 13-8-1 on the season — never established momentum.

First-period goals from David Pastrnak (PP) and Trent Frederic put Colorado in a 2-0 hole. Then a Pasternak breakaway score in the second made a comeback feel impossible without quality depth. Avs goalie Pavel Francouz didn’t get much help, either, in a 35-save effort. He stopped a Pastrnak penalty shot.

Andrew Cogliano scored the Avs’ lone goal cleaning up a rebound in the third period. Then Boston buried them with two goals scored within a 10-second span from Frederic and Jake Debrusk.

Colorado traveled to Boston with the NHL’S most productive power-play unit (32.7% conversion rate). But it failed to produce on all three chances.

The Avalanche proved undisciplined, again, with six total penalties. They committed seven infractions Thursday night in the Buffalo victory.

“It’s unfortunate. We want to stay disciplined,” defenseman Cale Makar said. “But at the same time, we have to trust our PK and get it done. Right now, we’re grinding out there.”

Lehkonen’s absence is especially concerning. The do-it-all winger brings needed toughness to the top line and power-play unit. He racked up 18 points (seven goals) this season prior to injury.

“When you have seven guys out from your starting lineup, it’s hard. Especially against a team like this,” forward Mikko Rantanen said. “But no excuses. We’ll try to work through it.”

Lehkonen’s injury appeared to take place in the first period, resulting from a net-front collision with Boston players, in which Lehkonen fell awkwardly to the ice. He was replaced on the top line by rookie Jean-luc Foudy, and on the first power-play unit by Alex Newhook. Bednar did not have an immediate health update on Lehkonen. The Avalanche, having lost two of their past three games, get a bounce-back opportunity Monday night in Philadelphia against a rebuilding Flyers team. Colorado just needs to stay healthy.

“Hopefully, we have reinforcements coming at some point without losing too many guys here,” Bednar said. “When the task is getting tough, you can’t fragment away from it.”