Torey Krug was apparently injured during the middle of the first period. The Bruins announced that he will not return. Using the shift chart from war-on-ice.com, we can see that he ended his shift a little early around the 12 minute mark:
Here's his last shift, as well:
Looks like he was skating hard but pulled up at the last second and then went off. Non-contact injuries are always scary. Krug has been playing well, hope he's alright.
Video courtesy CrzyCanucklehead