The status of injured defenseman Zdeno Chara and Matt Grzelcyk forced the Bruins into an unfamiliar, unsuccessful situation in Game 5. Their best chance to win on Sunday night and force Game 7 hinges largely on whether the defense corps is healthy enough to let them deploy their forwards as usual.

BOSTON -- Injuries. Suspensions. Officials’ calls. Officials’ non-calls.

 All have influenced the first five games of the Stanley Cup Final; all are bound to influence Game 6 on Sunday night at the Enterprise Center (8, NBC, WBZ-FM 98.5), where the Blues will try to win the first Cup in franchise history, and the Bruins will try to bring the series back to TD Garden for Game 7 on Wednesday night.

 Officiating gradually become the dominant topic of the series, from Blues coach Craig Berube’s complaint that his team was being unfairly targeted after the Bruins’ 4 for 4 performance on the power play in a 7-2 win in Game 3, to B’s coach Bruce Cassidy stating after Game 5 that officiating was giving the NHL a “black eye,” primarily because Tyler Bozek’s unpenalized trip of Noel Acciari ultimately allowed the Blues to score the decisive goal in a 2-1 decision.

 The Bruins can hope all they want for more favorable officiating in Game 6, but that won’t necessarily guarantee a win: Their power play is 0 for 5 over the past two games (both losses), and wasn’t even all that efficient in Games 1 and 2, when it went 2 for 10 on the strength of goals from its second unit. Throw out that 4 for 4 Game 3, and the power play is 2 for 15, 13.3 percent.

 Improved production, however, remains critical to extending the series, and their best chance to score more will be determined, ironically, by the health of their defense corps.

 Uncertainty over how much, and how well, Zdeno Chara could play Game 5 with a reportedly broken jaw, combined with Matt Grzeclyk not being cleared to return from a concussion, prompted Cassidy to play it safe and dress seven defensemen on Thursday. That meant pulling a forward from the lineup, and given the facts that his top line comprises most of the first power play unit, and that his third and fourth lines had generated most of the Bruins’ even-strength offense in Games 1-4, Cassidy stripped the second line of right wing David Backes.

 For a while it looked like it might work: The B’s outshot the Blues 17-8 in Thursday’s first period. They didn’t score, though, and after the Blues took and did well to hold a 1-0 lead, Cassidy increasingly focused on generating more scoring chances. To do so, he effectively broke up his third line of Marcus Johansson, Charlie Coyle and Danton Heinen -- despite it being arguably the Bruins’ most consistent line in the series to that point, but also because of that.

 “That line was good,” said Cassidy, who frequently used Johansson on the second line with David Krejci (no points in this series) and Jake DeBrusk, gave Coyle shifts at right wing on the top line with Brad Marchand and Patrice Bergeron, and moved David Pastrnak between those two lines. “(But) part of the thinking was also that the players (Johansson, Coyle, Heinen) were good.

 “That was a product of trying to get the right guys out there, and hopefully it worked. It didn’t. We’ll have to look at it again for Game 6.”

 If Grzelcyk, who skated on Friday with the Black Aces taxi squad and practiced on Saturday at St. Louis, can be cleared for Game 6, or if Cassidy can enter Game 6 with more confidence that Chara will be able to last throughout, he can return to using 12 forwards, most likely in their standard line configurations: Marchand-Bergeron-Pastrnak, DeBrusk-Krejci-Backes, Johansson-Coyle-Heinen and Joakim Nordstrom-Sean Kuraly-Acciari.

 “A lot of times, we do go back to that,” the coach said, “(but) some of it’s going to depend on health. … Twelve (forwards) and six (defensemen) is obviously our better formula. I mean, we did it probably 80 times out of 82 (regular season) games, and all playoffs. (But) we’re going to do what we have to do, depending on the status of a few guys.”

 Granted, going back to the lines that started the series means questions will resurface about the lack of productivity from the top two trios. They’ve bounced back from earlier dips this post-season, though, and have been consistent power-play contributors. Coyle’s line has been the Bruins’ best in the series, and Kuraly’s will face a Blues fourth line weakened by the one-game suspension of Ivan Barbashev.

 A game with the Stanley Cup on the line isn’t the best time to be forced into something new. The Bruins’ best shot is that they can use the same old players, in the same old configurations, that carried them this far in the first place.