Lehner has been out since spraining his right ankle midway through a 3-1 season-opening loss to Ottawa on Oct. 8. He was called up from the American Hockey League on Thursday, a day after stopping 35 of 39 shots in Rochester's 4-0 loss at Albany.
He went 1-2 and allowed 10 goals in three games.
The Sabres were counting on Lehner to be their starter in June, when they gave up a first-round draft pick to acquire him in a trade with Ottawa.
Buffalo hosts Boston on Friday and Washington on Saturday.
To make room on the roster, Buffalo demoted rookie goalie Linus Ullmark to Rochester.