Most around the Boston Bruins figured Charlie McAvoy would succeed Zdeno Chara as the team's bona fide number one defenseman someday.

With the Black-and-Gold captain's facial injury (reportedly a broken jaw) threatening to keep him off the ice for the rest of the Stanley Cup Finals, someday has come a lot sooner than anyone would've dared to guess.

As the Bruins gear up for a pivotal Game 5 against St. Louis on Thursday night at TD Garden, contingency plans to fill Chara's 6-foot-9 skates are being drawn up all over Beantown.

Do they bench a forward and play seven defenseman, limiting the minutes and exposure of Steve Kampfer and, possibly, rookie Urho Vaakaneinen? Does the potential return of Matt Grzelcyk, who skated in a non-contact jersey at practice Wednesday, make that moot? Could Chara possibly play with the proper shielding in front his damaged face?

No one that steps into the lineup can fill in for Chara. More likely, the captain's minutes will be somewhat distributed between the other regulars, and his primary responsibilities would fall to his 21-year-old partner, McAvoy.

These next two, or three, games could tell us a lot about McAvoy, drafted in the middle of the first round a few years ago. He's widely viewed as one of the best defensemen in his age group in the NHL, a future Norris Trophy contender and possibly a future captain.

He's also a restricted free agent this summer, reportedly seeking a deal somewhere in the $7-8 million per year range that would make him Boston's highest paid player. Take the B's defensive corps on his back and win the Stanley Cup over the next week and he'll get all that and more.

But if he falters? Remember when Tyler Seguin took the fall for a bad Finals in 2013, the comment 'If he gave us half of Patrick Kane we win the Cup'? If the B's lose and the salary negotiation gets ugly, it's not hard to plug in 'If he gave us half of Colton Parayko, we win the Cup.'

The great thing if you're wearing Black-and-Gold is that there's no reason to think McAvoy won't rise to the occasion.

This is a player that made his NHL debut during the Stanley Cup playoffs three years ago, straight out of Boston University, and led the team in minutes played. He's 36th in the league among defensemen in points per game over his two regular seasons — and 50 of his 60 career regular season points have come 5-on-5, the area where the Bruins need to improve the most to get back in control of the Finals.

"We need the breakout to be smoother, more crisp," said McAvoy, explaining that if the Bruins get the puck out of their zone quickly, St. Louis won't be able to hem them in and forecheck for 30 or 40 seconds at a time.

Given the breakout is one of McAvoy's best skillsets, an increased work load for him stands to benefit Boston. Whether he's paired with a defense-minded guy like John Moore or possibly reunited with ex-BU partner Grzelcyk, he'll be without the Chara safety net.

"He knows he won't have his big brother back there," coach Bruce Cassidy said.

At every level of his young career, McAvoy has been better under the brightest lights. He was an All-Tourney selection when Team USA won gold at the 2017 World Junior Championships. He's been good in all three playoff runs for Boston, led the World Championships in defenseman points while helping the Americans win bronze last spring and was an All-American on Commonwealth Ave. as a Terrier.

Thursday night's Game 5 will be his 39th career Stanley Cup playoff game, before his 22nd birthday. That's a remarkable number made even more impressive by the 24-plus minutes a night he plays on average — and he's ready for more if called upon.

"We're a well conditioned hockey team," McAvoy said. "Whoever is in there can play as many minutes as needed."

Without Chara, the future for the Boston blue line is now. All along we've thought the future looked pretty good. But with a hard and heavy St. Louis team standing between the Bruins and the Stanley Cup, we're about to find out just how bright that future may be.

"It's best of three for the Cup. If you'd told me before the season we'd be in this position, I'd have been elated," said McAvoy. "Now we've got to make the most of it."

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You can contact Matt Williams at MWilliams@gloucestertimes.com and follow on Twitter @MattWilliams_SN