Zdeno Chara can't play forever. Sure, he may have another few years left in him, probably has at least one or two more high-quality campaigns in those tree trunk-like legs. And yes, in the here and now, Chara can still provide a major impact on any game, on the season as a whole. But at some point in the not-too-distant future, Chara, 39 in March, the longtime Boston Bruins captain and grizzled veteran leader, won't be patrolling the B's backend anymore. And when that day comes, there could be trouble in beantown. Why? Because there doesn't seem to be a ready replacement for Chara - physically or emotionally - in the Bruins' defensive corps (not that there are many 6-foot-9 defenders with booming slapshots and a long reach in the NHL or the world). And to hear longtime voice of the Bruins, Jack Edwards, tell it, this is the biggest concern facing the franchise and is likely what keeps new Boston GM Don Sweeney awake at night.

"The D core is in a crisis right now as far as a mid- to long-term outlook," Edwards said recently, as part of an in-depth interview with Sportsnet.ca. "There's no alpha dog on the way. Colin Miller is not going to be an alpha dog. He'll be a top-4 defenseman, no doubt, but right now he doesn't look like a guy who's going to turn into Duncan Keith."

That's a pretty sour outlook on Miller, a fifth-round pick of the Los Angeles Kings, added by Boston as part of the Milan Lucic deal with L.A. this offseason, but if there's anyone qualified to offer such an assessment, it's a guy that watches Miller play on a nightly basis. At just 23, there's still plenty of time for Miller to add to his overall play. He's operating on the second unit at present alongside Dennis Seidenberg. He's got two goals and eight assists and is averaging 15:45 minutes of ice time per night through his first 19 games at the NHL level.

Yes, at this point you'd be hard-pressed to find anyone to suggest that Miller can become what Chara is and has been to the Bruins, but it's early - perhaps too early.

Still, there's no denying the important or accuracy of Edwards' overall point. After the Dougie Hamilton trade this offseason - a player Edwards said may have been too "aloof," during his time with the franchise, but was in no way a problem in the locker room - there is little in the way of depth, organizationally, on the Bruins blueline.

Torey Krug has made the fulltime jump to the NHL after first debuting in October 2014. He's up to 22:01 minutes of ice time per night, though his offensive numbers to start the season - one goal, 12 assists - are a bit lower than he's managed the last two years. That's likely due in large part to the team trusting him with more defensive responsibility, but the fact remains that he's a quality player who offers a high offensive upside. The question facing Krug is whether he's ever to take his game to the next level and employ both his offensive skill and a defensive mentality simultaneously.

Zach Trotman, playing alongside Chara on the Bruins' top pairing, has offered little impact offensively over the past 10 games. The former seventh-round pick is still just 25, but his overall ceiling is, seemingly, low. Along with Chara, he's been solid in a kind of shutdown defensive role, but he's not a game changer and doesn't possess the leadership skills of his partner.

Joe Morrow almost made the Pittsburgh Penguins out of his first professional training camp ahead of the 2011-12 season, but he's fallen back to Earth since being obtained by the Bruins as part of the Tyler Seguin trade. Like Trotman, his ceiling simply isn't high enough to be considered a potentially elite player in the future.

Jakub Zboril, one of three first-round picks for the Bruins in the 2015 NHL Draft, has the highest upside of any prospect in the Boston system. He already plays what is considered to be a "sound defensive game," which he pairs with a strong shot and some serious offensive skill, but at just 18, it's far too early to say with any confidence what he is or what he can become.

In short, the Bruins have a fair amount of potential in the pipeline, but few players that can be counted on to become contributors at the NHL level and even fewer that could rightly be expected to provide the kind of impact play from the blueline the franchise has enjoyed during Chara's reign.

The "alpha dog," as Edwards lovingly dubs Chara, is sure to remain with the team for the next couple of seasons. But beyond that, there are many questions, and few answers, regarding the Boston defense.