The Bruins announced on Tuesday that Patrice Bergeron will miss at least four weeks with a rib and sternoclavicular injury. He joins captain Zdeno Chara, who is expected to be lost for the same amount of time with a knee injury.

The Bruins played on Saturday night at Arizona without five of the six defensemen who are virtually assured of being among the Top 6 when they’re healthy. It wasn’t always fun to watch, but the B’s won, 2-1.

On Wednesday night in Detroit (7:35, NESN, WBZ-FM/98.5), they’ll be without only one of their forwards, but it might be even harder to win without him.

The Bruins announced on Tuesday that No. 1 center Patrice Bergeron suffered a rib and sternoclavicular injury during Saturday’s 1-0, overtime loss at Dallas, and won’t play at least until he’s re-evaluated in approximately four weeks.

Losing Bergeron isn’t losing a single forward. His absence costs them their most productive player (26 points in 19 games), their best set-up man (No. 1 with 17 assists, nine on power plays), their busiest and best on faceoffs (436 draws, 55.7 percent won), and the forward who plays the most (1:59 per game) in shorthanded situations.

“You can’t replace a guy like that,” coach Bruce Cassidy told reporters after a practice on Tuesday in Detroit where there was at least some good news: Defenseman Kevan Miller is expected to return after missing 13 games with a hand injury, and fellow blue-liners Charlie McAvoy and Urho Vaakanainen practiced for the first time since they suffered concussions. “It’s 20 minutes a night that’s going to get moved around into some different situations.”

Bergeron’s injury came directly on the heels of a knee ligament injury captain Zdeno Chara sustained last Wednesday at Colorado. He, too, won’t play until after a re-evaluation in roughly four weeks, leaving the B’s without their best defensive forward, best defensive defenseman, and two longest-serving leaders until close to Christmas.

“It’s obviously a big loss,” said center and assistant captain David Krejci, who will move up to a new No. 1 line and maintain his position on one of the Bruins’ power-play units, but rarely gets involved in penalty-killing situations. “Both of those guys are experienced, and they’re leaders on this team. You’re not going to replace them.”

Perhaps because the Bruins have managed relatively well despite their defense corps being shredded by injuries (through 20 games, eight defensemen have missed at least one game), that situation seemed merely startling. They’ve gone 7-4-2 in the games both Miller and McAvoy have missed, 2-1-1 since Brandon Carlo (upper body) was added to the injury list, and surrendered only one goal to the Coyotes with five regular blue-liners out.

Spending four weeks without Chara and Bergeron, however, qualifies as daunting -- especially when the short-term schedule is considered. The Bruins, who sit fifth in the Atlantic Division and hold the second Eastern Conference wild-card slot, host the Penguins on Friday night before visiting the surprising Canadiens on Saturday and the division-leading Maple Leafs on Monday.

“In these situations, where we’ve had a lot of guys injured … you can view it as a negative thing,” said Miller, whose size, experience and leadership abilities will all be welcomed back, “or you can use it as kind of a team-building thing, to rally around the group that’s playing.”

Cassidy is left to try to organize that group, which includes pure rookies Jeremy Lauzon, Connor Clifton and Jakub Zboril on defense (one will be scratched on Wednesday), and Jakob Forsbacka Karlsson at center. Among other moves, the coach will execute a long-in-the-making plan to use goals leader David Pastrnak (17 in 20 games), Bergeron’s right wing, with Krejci, and move Bergeron’s left wing, Brad Marchand, onto a second line centered by Joakim Nordstrom, who was signed as a free agent last summer to be a fourth-line wing. David Backes (15 games, one assist), recently relegated to right wing on the fourth line, will replace Bergeron on the top power-play unit.

“It’s to be determined, to be honest with you,” Cassidy said. “We’ll see how it all plays out. We’re going to need a little bit of everything from everybody.”