Kelly was transported to Massachusetts General Hospital and will undergo surgery on Wednesday, according to the team.
"You hope we can somehow compensate for his loss, but I think people are going to realize how important he is to our hockey club, not just on the ice but in the dressing room and around the team," said Bruins coach Claude Julien. "He's a guy that is extremely respected by his teammates and on the ice he's given us everything we've asked for."
At 1:40 of the first period, Kelly was skating in front of the benches when he got tangled up with Stars forward Colton Sceviour while both were trying to gain control of the puck. Kelly fell awkwardly and twisted his left leg. He was in clear discomfort and grabbed his left knee. Team trainer Don DelNegro tended to Kelly, and after a few minutes on the ice, Kelly was carried off and taken to the dressing room.
"It's huge," said assistant captain Patrice Bergeron of Kelly's loss. "He's a veteran that is really important for us on and off the ice. He brings so much to the team, especially as a leader. As soon as I saw him go down it didn't look good and kind of felt like it was going to be a pretty bad injury and unfortunately that's exactly what it is and I'm definitely thinking of him right now. Hopefully the pain is not too bad."
During the 2013-2014 season, Kelly missed 25 games and the entire postseason due to injuries. In 2012-2013, he missed 14 games due to a broken tibia. Now, he's gone for the remainder of the 2015-2016 regular season.
Kelly, 34, will be an unrestricted free agent on July 1. He's making $2.5 million this season and finished with two goals in 11 games.