Mittelstadt (upper body) won't return to Thursday's game versus the Canadiens.
Mittelstadt was hurt in the second period and made a brief return to the ice before being ruled out in the intermission. The 22-year-old had the first chance to fill in as the No. 1 center this year with Jack Eichel (neck) unavailable, but it's unclear if Mittelstadt will miss any time. The Sabres' next game is Saturday versus the Coyotes.