The forward had surgery Saturday after he was hit in the head by an errant shoot-in during the first period of a 5-2 win at the Washington Capitals on Thursday.
"It's a loss," Sabres coach Don Granato said Saturday. "He's a leader, he's helped us as coaches in the transition, he's such a great communicator to us, to the teammates. Everybody looks up to him. So we definitely lose a big portion of that.
"He's a guy that's around all the time and I'm sure he'll still be helping us in many respects. And then the other side of it is, you think about it, I'm glad this didn't happen any sooner. 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."
Okposo scored 13 points (two goals, 11 assists) in 35 games this season. The 33-year-old has two seasons remaining on a seven-year contract he signed with the Sabres on July 1, 2016.
Buffalo (11-25-7) is last in the eight-team MassMutual East Division and will be eliminated from playoff contention with a regulation loss to the Pittsburgh Penguins on Saturday (3 p.m. ET; NBC, NHL.TV) or a loss of any kind combined with a New York Rangers win of any kind against the New Jersey Devils on Saturday.
NHL.com independent correspondent Heather Engel contributed to this report
