BOSTON (CBS) — The Bruins’ vaunted quartet of top defensemen took a major hit Saturday when general manager Peter Chiarelli decided to cut bait with Johnny Boychuk before the season rather than go all year with the blueliner in a lame-duck contract situation.

Chiarelli traded Boychuk to the New York Islanders for the Philadelphia Flyers’ second-round pick in 2015, the Islanders’ second-round pick in 2016 and a conditional third-round pick in 2015.

A top four that was expected to include Zdeno Chara, Dougie Hamilton, Dennis Seidenberg and Boychuk during the Bruins’ attempt to prove last year’s second-round playoff ouster against the Montreal Canadiens was an anomaly now includes Chara, Hamilton, Seidenberg and a question mark.

Everyone knew the Bruins weren’t going to be able to sign Boychuk after this season as an unrestricted free agent. A right-handed shot who’ll be 31 by next summer, Boychuk should command at least $5.5 million per season on a long-term deal from a team with that type of cap space. The Bruins, who are struggling to get below the salary cap ceiling for this season, were not going to be in a position to retain Boychuk because of their current salary commitments and what they’re going to have to give a free agent class highlighted by defenseman Dougie Hamilton (restricted) and center Carl Soderberg (unrestricted).

Chiarelli spoke about the trade “globally” as being the first of several steps that he’ll make over the course of the season to make sure he has a championship-caliber team by the day after the trade deadline. He also conceded that the trade “doesn’t make us better now.” That’s not exactly a ringing endorsement of his own work.

Obviously the Bruins are banking on drastic improvement from at least one player in their remaining defense corps. At first, it appears that Seidenberg, who is coming off major knee surgery is 33 and is typically a slow start, will stay on the left side and Kevan Miller will move into the spot next to him. Miller is entering just his second NHL season and averaged 17:27 of ice time last season. He was used mostly in a third-pair role and there’s no telling what having his minutes increased near Boychuk’s 21:11 or facing tougher competition will do to an undrafted signee.

Veteran Adam McQuaid might move up, but in his four seasons he’s never been more than a third-pair defenseman, even when he wasn’t coming off the rash of injuries that limited him to 30 games in 2013-14.

Then there are the options the Bruins have if they want Seidenberg to play on the right side, where he’s done well in past years for them paired with Chara. Torey Krug, who’s obviously currently a little behind because it took him a week of training camp to sign a new contract, proved he could handle some difficult assignments in the playoffs last season. His ice time increased, but his effectiveness didn’t. Of course, the playoff run was short circuited and we never found out if he could hold up through four rounds in an expanded role. Can the 5-foot-9, 180-pound defenseman hold up under increased minutes and improved opposition over 82 games? Not likely.

So far Matt Bartkowski has somehow survived the Bruins’ training camp cuts. The waiving of David Warsofsky and trade of Boychuk left Bartkowski as the seventh defensemen, a role he handled well last season when he wasn’t playing. Once he got in the lineup, well … I don’t have to tell you. And just a week ago, here’s what Bruins coach Claude Julien had to say about Bartkowski.

“Defensively, as you’ve heard before, he struggles a little bit in his own end,” Julien said. “He gets beat off the walls. Guys aren’t perfect. We expect guys to work on certain things. … Doesn’t mean we don’t like him, doesn’t mean he’s not good, but more that those are the issues that we ask this specific guy to work on. Get a little bit better in the D zone as far as not losing wall battles and not letting guys beat him back to the net.”

So basically, the coach admits Bartkowski is not a viable defenseman. You could argue that Bartkowski is now one injury away from being in the top four on a team that thinks it’s going to bring the Stanley Cup home again.

From the time Chiarelli took over and Chara signed, the Bruins had to work their way through to a championship-caliber defense corps. From Paul Mara and Nathan Dempsey to Matt Hunwick, Matt Lashoff, Aaron Ward and Steve Montador it took several years. Now with a team that has a lot of guys entering the final season of their contracts and looks like it might be gunning for one last hurrah as a group, Chiarelli has pulled a thread that could dismantle his defense corps until he makes a move to sew it back up.

Maybe there’s another shoe waiting to drop. I’m still not convinced these are the top nine forwards the Bruins are going to go through the season with. They have to add a right-handed shot at some point, especially if the Jarome Iginla-less power play doesn’t click early on.

The roster surely looks like it’s in flux. There should be more moves coming. Teams in transition don’t usually finish the season with the Cup. Maybe Chiarelli can break that trend.

Matt Kalman covers the Bruins for and also contributes to and several other media outlets. Follow him on Twitter @TheBruinsBlog.

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