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."
After a delay of about seven minutes, Hackett was replaced by Connor Knapp, who made his NHL debut.
"Yeah, yeah you never want to see that happen," said Knapp, who stopped six of seven shots. "Obviously it was bad. You know all the guys feel pretty terrible about something like that happening, especially with everything that's happened already. Definitely not the way you want to play your first NHL minutes seeing [Hackett] go down like that; it's tough."
The Sabres have lost almost 300 man-games to injury this season, and three players in addition to Hackett left the game Saturday: defenseman Christian Ehrhoff (concussion), and forwards Chris Stewart (ankle) and Brian Flynn (lower body)
The Sabres are 30th (last) in the NHL standings.
"I've never seen anything like that," Buffalo forward Drew Stafford said. "You know to go through that on the bench that's quite a bit of a challenge. Just add it to the list this year. I mean we don't want to use any excuses at all for what's going on, but I don't know if that's our ninth goalie or whatever it is and we are playing basically with two lines for the last half the period."
Hackett has a 1-6-1 record, 3.10 goals-against average and .908 save percentage in eight games for the Sabres this season. He played 13 games for the Minnesota Wild in the prior two seasons.