The Bruins said Vatrano was injured training for this season. He'll have surgery Monday.
Vatrano, 22, was expected to compete for a spot on one of the Bruins' top two lines.
"Obviously it's a blow," Bruins general manager Don Sweeney said. "Frankie looked at [training camp] as an opportunity; we all did. How that was going to play out remained to be seen, but he certainly was going to be put in a position to try to grab hold of it, so he's disappointed. We are, as well.
"We look at it, as all injuries are, as a setback. But the doctors feel very good that three months from now he'll be returning to play and hopefully moving forward."
Vatrano had eight goals and 11 points in 39 games with the Bruins. Playing with Providence, the Bruins' American Hockey League affiliate, he had an AHL-best 36 goals in 36 games.
