The 32-year-old Okposo joins a growing list of injuries for the last-place team that is one loss from being mathematically eliminated from playoff contention and matching an NHL-record 10-year postseason drought. The Sabres are already minus captain Jack Eichel, starting goalie Linus Ullmark and defenseman Jake McCabe.
A 14-year NHL veteran who is completing his fifth season in Buffalo, Okposo has two goals and 11 assists in 35 games this year.
Okposo has filled a valuable leadership role and has been a stabilizing voice for a team that went through an 18-game winless skid this season, which matched the 14th longest in NHL history.
"Everybody looks up to him, so we definitely lose a big portion of that," interim coach Don Granato said before Buffalo hosted the Pittsburgh Penguins. "It's bad that it's happened and he's missing the season, but certainly I'm grateful we've had him to this point because he's done an awful lot for us."