It’s no surprise that improving the Bruins defense is one of the top priorities for general manager Don Sweeney and the rest of Boston’s management group.

This past season the Bruins were 20th in the league in goals allowed per game, allowing 2.78 goals per contest. Defensively, the Bruins were slow and overall pretty inexperienced as a rotation of young defenseman came in and out all season long.

Focusing more on taking the space given to them and moving the puck from their own end in transition was a key part of the Bruins new defensive mindset. But the lack of a true puck-moving defenseman was a hindrance to the Bruins’ newest defensive game plan.

On Tuesday, the Bruins announced they have come to terms with free agent to be, Kevan Miller on a four-year $10 million extension. With the signing of Miller, the Bruins have just four defensemen currently under contract. Zdeno Chara, Adam McQuaid and Dennis Seidenberg make up the other three.

Miller had his ups and downs last season, but Sweeney sees many positives in Miller’s game, which led to his return to Boston.

“Kevan brings incredible character. His signing provides us with the necessary depth on our defense that all teams need. His relative low-mileage, having just played 160 games, we identified that we think Kevan has room for continued growth and development,” Sweeney said Tuesday afternoon via conference call.

“We certainly saw that in his play this year when he had an expanded role. Relative to the free market place, very, very comfortable with where Kevan fits into our group, and this provides us with the opportunity to explore the marketplace in every way, shape, or form, in having Kevan signed.”

The Bruins now have just under $14 million committed to the four defensemen under contract. With Miller back on the books, where do the Bruins go from here?

“Well, I think we need to look for another transitional D-man. We’ve had talks with [restricted free agent] Torey [Krug] and we’ll find, whatever term that ends up being, we’ll find a contract for him. But we’re looking for balance,” Sweeney noted. “We’re also looking for players like Colin Miller to take the next step.”

Exploring the trade market including that of Seidenberg and McQuaid is also an option Sweeney wouldn’t rule out.

“I’ll explore whatever I have to do, in every way, shape or form to continue to improve the club and find the blend and the balance that we need,” added Sweeney. “I said that in my opening statement at the end of the year that we’re just going to continue to look to improve our club in every way, shape or form.”

Improving the team may be easier said than done, especially on the back end. Defensively, there’s not much out there.

Sure, the Bruins could improve their defense via trade, but at what cost?

The Bruins are in desperate need of two, top-pair defenseman, something that won’t come cheap in the trade market. The Bruins do have two first round picks in next month’s entry draft that can help get them that top defenseman they so badly need, but it will obviously take much more than that.

The salary cap is also not an ally to the Bruins. They have used just over $51 million of cap space for 14 players, and don’t forget, Brad Marchand is a free agent next summer.

How different the Bruins’ blueline will look come October remains to be seen, but the bottom line is simple: the Bruins need a mix up on their blueline or else things will turn south on Causeway Street, and turn south quickly.