BOSTON -- This is not how the Boston Bruins wanted to start the 2015-2016 season.

General manager Don Sweeney announced Wednesday that veteran defenseman Dennis Seidenberg will have surgery Thursday to repair a lumbar spine disc herniation and will miss approximately eight weeks.

“It’s definitely a setback,” Sweeney said.

Prior to training camp, Sweeney said he was pleased with the organization’s defensive depth. Now that Seidenberg will be sidelined, the GM did not dismiss the idea of a possible trade to acquire another veteran defenseman. Sweeney said there are internal candidates to fill the void, but the trade market remains an option.

“Only under the right circumstances,” Sweeney said. “It’s got to be the right fit for us, relative to the guys that we have. We felt we had very good depth, albeit some inexperienced, but now they’re getting an opportunity and hopefully they can take advantage of it.”

Boston’s blue line has Zdeno Chara, Adam McQuaid, Torey Krug and Kevan Miller. With Seidenberg out, newcomer Matt Irwin and prospects Colin Miller, Zach Trotman and Joe Morrow have an opportunity to step up.

“It’s never good news when one of your teammates, a steady defenseman, will be out of the lineup for a period of time,” Chara said. “We don’t know how long it’s going to take but we all wish him a fast and healthy recovery.”

Seidenberg participated in captain’s practice prior to training camp and had no symptoms, according to Sweeney. Seidenberg has no idea how the injury occurred. He woke up one morning two weeks ago with tightness in his back and immediately notified the team’s medical staff. The first course of action was rest, and while it did not get worse, the discomfort did not subside so the medical staff thought it best to have the surgery now.

Even though Seidenberg played all 82 games last season, he was still rebounding from knee surgery after he tore both his MCL and ACL in December 2013.

“We certainly had high expectations with how he would fit into our group,” Sweeney said. “A lot of players do take, in any sport, a long time to recover from an ACL. The way Dennis is built to get back and do things as hard as he can it’s no surprise he was able to play 82 games last year, despite coming off the surgery. He felt good about where his training was at overall and he was excited about the year, as we were with how he was going to fit into our group.”

With Seidenberg sidelined, the Bruins lose a big, strong, steady defenseman. Fortunately it’s still early enough in camp where another player can step up in his place.

“I’m always motivated to make that jump to the next level, but you’ve got to focus on your own game and push forward every day and continue to improve,” said Colin Miller, who was acquired from the Los Angeles Kings in June as part of the Milan Lucic trade.

Miller has played the last two seasons for the Manchester Monarchs, the AHL affiliate of the Kings, but has no NHL experience.

Trotman, 25, played 27 games with the Bruins last season and recorded one goal and four assists for five points. Regardless of Seidenberg's injury, Trotman said he’s already motivated to earn a spot on the roster.

“That urgency is already there,” he said. “You have to go with the same mentality every day. [An injury is] one of those things that’s unfortunate and you can’t control, so you have to have the same mentality of just competing day in and day out.”

Irwin, 27, has played the last three seasons for the San Jose Sharks and signed with Boston as a free agent this summer. He had eight goals and 11 assists for 19 points last season with the Sharks.

"I like our group,” Irwin said. “We’ve got a lot of D that are capable of playing. Everyone can play at this level and it’s good to push each other at practice and in games to be the best you can be, leave your impression on the coach and see where it falls. A lot of us bring different intangibles to the game and when you’re given the opportunity you’ve got to take advantage of it.”

Coach Claude Julien said he plans on keeping seven or eight defenseman on the roster to start the season.

“It’s important for me right now to focus on what I’ve got and to make sure I do the best to make it work,” Julien said. “It’s always part of the game when you have players get injured throughout the season. It’s unfortunate it’s happened, but I’ll move forward with the guys that we have and there are a lot of guys eager to prove that they belong here.”