Coach Dan Bylsma revealed the timetable on Sunday after Lehner underwent a series of tests. Lehner was hurt in the second period of Buffalo's season-opening 3-1 loss to Ottawa on Thursday.
He was untouched while circling his own net and before suddenly going down in his crease.
Lehner was supposed to play a key role with the new-look Sabres this season. They traded a first-round draft pick to Ottawa to acquire the 24-year-old goalie and forward David Legwand in June.