The season is only one game old and the Bruins already are down three players. Patrice Bergeron (lower-body injury) and David Backes (diverticulitis) missed the opener against Nashville on Thursday, and now fourth-line winger Noel Acciari has joined them on the sideline.

Acciari, also one of the team’s go-to penalty killers, suffered a left index finger fracture during the 4-3 victory over the Predators when he blocked a P.K. Subban slap shot.

Acciari underwent surgery, performed by Dr. Matt Leibman, to repair the finger and is expected to miss approximately six weeks.

Bergeron skated on his own before Saturday’s practice at Warrior Ice Arena, moving deliberately. Coach Bruce Cassidy wouldn’t rule him out for Monday’s game against Colorado, and they expected to reassess the possibility on Sunday.

Also on the ice were defenseman Torey Krug (fractured jaw), wearing both a visor and a shield, and forward Austin Czarnik (illness). Both were cleared to practice, but Cassidy would only say they were a step closer to playing and that a decision would be made Monday morning. Krug practiced alongside Adam McQuaid, while Czarnik spent time on a line with Frank Vatrano and Matt Beleskey.

Krug, who was able to skate during his recovery from the Sept. 19 injury, said his first day at a full practice went well but was pretty much devoid of contact. “I tried to force some contact to see how it felt,’’ he said with a grin. “We’ll see how tomorrow goes and then just play it by ear every day.’’

Watching the new, faster version of the Bruins from the stands has been frustrating for the mobile defenseman.

“That’s the game that I’ve been trying to play since I came into the league,’’ Krug said. “It only took five years for our team to get on the same page. It’s exciting to be a part of something where everyone’s aggressive, everyone’s assertive. It’s a fun time to be a part of this group.’’

It was the third day of practice for Czarnik, who said he was surprised that both his legs and lungs felt good.

“Real good day for me,’’ he said.

Czarnik was reunited at practice with a former Providence linemate in Vatrano.

“We both like to play up-tempo style,’’ Vatrano said. “[Czarnik is] a guy that sees the ice really well. For me, I like to get open for him and try to get in the corners and make some plays and get him the puck as much as possible.’’

The loss of Acciari means Cassidy will have to do more mixing and matching with his forward combinations. He said Acciari’s absence will be felt.

“He gives some flavor to that bottom of the lineup in terms of physicality,’’ said Cassidy. “He had good chemistry with [Riley] Nash. He plays a straight-line game and with [Tim] Schaller and [Sean] Kuraly, gave us a hard-to-play-against line with some ability to chip in offensively.

“[Acciari] is a tough one [to lose]. He’s a guy that will put himself in harm’s way at a moment’s notice.’’

The penalty kill is now without Bergeron, Backes, and Acciari as possibilities. Cassidy said Beleskey will probably get a few more reps, and perhaps Kuraly. Cassidy also said he wanted to stay away from using his younger forwards (Jake DeBrusk, Anders Bjork) on the PK, as they “had their hands full.’’

“We’re obviously going to have do it by committee,’’ Cassidy added.

Nash, frequently Acciari’s partner killing penalties, felt bad for his teammate.

“It’s tough for a guy that’s earned his way the last three months, the last year,’’ said Nash, who played with Schaller and Kuraly after Acciari left Thursday’s game.

“Tough break for him. We had established some pretty good chemistry, but you’ve got to try and figure it out with other guys, too.’’

The Bruins’ most productive line in the opener was assembled in the middle of the game. David Krejci had three assists, DeBrusk had his first NHL goal and an assist, and Bjork had an assist.

“They may very well start like that,’’ Cassidy said, adding, “how they finish is dependent on situations in the game. Obviously they played well together, they read off each other.

“[Krejci’s] really skating well. He’s 100 percent healthy, that helps. He’s not chasing the game. Last year coming off the injury he was playing a little bit of catch-up. This year he seems to have a lot of juice, and playing with those guys he’d better.

“You hope that chemistry develops, and maybe we’ve got something here. It’s one game and we don’t want to get ahead of ourselves.’’

“With those young guys, it was fun,’’ said Krejci. “They make plays. Jake scored his first goal and Bjorkie had an assist. That will give them confidence. You could see they were making a lot of plays after that first goal.’’

Bjork is ready to learn from his experienced centerman.

“Krejci plays with a lot of poise,’’ the rookie said. “He plays really hard, but he has patience as well. I saw how much success he had using that poise and patience. It’s definitely something I’m trying to emulate and learn from.’’