There are many reasons the NHL is the fastest league in sports. Just over an hour before Saturday’s game, Bruce Cassidy believed he had defensemen to spare.

“It’s not easy,’’ the Bruins coach, about 75 minutes before puck drop, said of telling players like Adam McQuaid he would be out of uniform. “Worst part of your job is telling a good player, good person, that he’s not suiting up. But it is part of your job. You’ve got to do it. We try to be as honest as we can and worry about the next game.’’ 

Soon after those words tumbled from Cassidy’s mouth, some other impolite ones probably followed after he learned Charlie McAvoy’s workload would be disrupted. McAvoy could not complete his only shift. He went awkwardly into the boards. Then he hit the deck after being tripped by Brendan Gallagher. With a helpful shove from Anton Khudobin, McAvoy skated off the ice. After a pause in the tunnel with trainers at his side, McAvoy retreated to the dressing room for good. His night lasted 37 seconds.

After the 2-1 overtime win, Cassidy did not have any clarity on the nature of McAvoy’s lower-body injury and whether it would be a long-term concern. The Bruins are off Sunday. They practice on Monday and host Detroit on Tuesday. 

The Bruins announced a likely end to McAvoy’s night even before Gallagher’s game-opening goal at 9:16 of the first period. That the team promptly shut McAvoy down before the rookie even had a chance to address whatever kink he suffered indicates it was no minor boo-boo. No coach likes to be down a defenseman for 60-plus minutes.

“Clearly we had some issues moving the puck out of our zone,’’ Cassidy said. “Charlie’s a transporter. He’s a mover, passer, all of the above. Offensive zone, he can create some space. He can get some shots through, make some plays. Then it wears on you from the opening shift, defending. That’s 23 minutes a night that you’ve got to now parse out for the full 60, 59 minutes, with five guys.’’ 

The Bruins are already down Patrice Bergeron, one of their five most important players. It is unknown how much time Bergeron will miss because of his fractured right foot. The Bruins initially planned to re-examine the No. 1 center two weeks after the injury. That timeline is likely to represent only a segment of Bergeron’s absence.

Now they are missing McAvoy, who shares company with Bergeron, Brad Marchand, Zdeno Chara, and Tuukka Rask as the team’s five-player lead group. McAvoy does everything for the Bruins. He matches up alongside Chara against every opposing top line. He mans the point on the No. 2 power-play unit. When necessary, McAvoy is available for penalty-killing duty. The 20-year-old does everything hard, whether it’s skating the puck out of the zone, snapping an outlet pass onto a teammate’s blade, or giving the Bruins extra points with overtime or shootout strikes.

The Bruins have the luxury of a month-plus of near-irrelevant regular-season play. Such was the case against Montreal. The Canadiens are out of it. The Bruins are practically locked in to a first-round meeting with Toronto. It is a sad time when the two rivals clash in March with no playoff consequences on the line.

So if the Bruins are to be without McAvoy, now would be a good time. Cassidy has options. Brandon Carlo (20:04 of ice time) took some of McAvoy’s five-on-five shifts next to Chara. Kevan Miller shook off a first-period turnover to log 21:41 of play. Nick Holden, the left-shot defenseman acquired from the Rangers, took some right-side shifts. Holden also replaced McAvoy on the second PP unit. Holden’s slap-pass landed on the stick of Jake DeBrusk for the game-tying redirection at 17:15 of the third.

 McQuaid, a healthy scratch for three of the last four games, is ready to resume his broad-shouldered game on the right side. The 31-year-old McQuaid is a pro. When the defense is at full power, McQuaid is probably No. 8 on the depth chart. He may not agree with this ranking. But McQuaid has been around long enough to understand the depth and breadth of his competition and how the Bruins are better for it. 

Last summer, McQuaid did not enjoy being left unprotected in the expansion draft while the Bruins declared Chara, Krug, and Miller off limits. But he has been around long enough to understand the nature of the business. So if he’s needed on Tuesday, the hammer-hard McQuaid will be sure to express his excitement of playing against the Red Wings unlucky enough to tread into his territory. 

Matt Grzelcyk has missed the last two games because of a lower-body injury, although the rookie was well enough to participate in pregame warmups Saturday. It’s possible, if the Bruins were desperate for Saturday’s two points, Grzelcyk could have played. 

None of the options is as good as McAvoy. His most critical work takes place deep in his own zone. But he affects plays 200 feet away because of how rapidly and accurately he moves the puck out of danger, whether by skating it out of harm’s way or by passing it into safer spaces.

After the Bruins landed Holden on Feb. 20, they had nine healthy NHL defensemen. That crew included Paul Postma, the perpetual spare defenseman who is now finding his legs again in Providence.  

Just over a week later, they are down to six. That’s life at NHL speed.

Fluto Shinzawa can be reached at fshinzawa@globe.com.