Coach Ron Rolston announced Thursday that Leino is out indefinitely after he was hurt in Buffalo's season-opening 2-1 loss at Detroit a day earlier.
Rolston said Leino was hurt in the first period, but continued playing. The severity of the injury wasn't revealed until Leino was examined before practice Thursday.
Leino played in just eight games last season because of a hip injury. He also missed 10 games the previous season with a broken foot after being signed to a six-year, $27-million contract.
Forward Marcus Foligno (shoulder) returned to practice Thursday, but is not expected to play until next week.
Buffalo opens its home schedule against Ottawa on Friday.