The Bruins defenseman was skating toward the net to receive a pass when he collided with Sabres defenseman Jamie McBain, went airborne, and fell on Hackett's right leg.
The goalie was unable to get up and medical personnel were required to take him off the ice with 11:35 to go.
"I haven't had a chance to watch it back. Honestly I don't even know what happened," Krug said. "I just felt like my feet got kicked out from under me and I landed on his leg. For a second I thought he was OK and then he started screaming. So I feel bad. It's just a tough play."
Hackett, 24, was scheduled to travel back to Buffalo with the team.
"It doesn’t look good right now," Sabres coach Ted Nolan said. "We're going to wrap him up as best as we can and get him back home, get our doctors to look at him, but it doesn't look too good right now."