The team issued the update without revealing the nature of the injury Wednesday, a day after Ullmark was hurt in a 5-2 loss to Ottawa.
Ullmark was untouched in his crease while tracking Ottawa forward Artem Anisimov circling the net. Ullmark’s left skate slipped out from under him, and he fell back with his right leg bent under him.
He was unable to put any weight on his right leg while being helped off the ice with 9:32 remaining in the third period and after stopping 30 of 33 shots. Ullmark has a 16-14-3 record and has held the starting role since mid-November after spending the first six weeks of the season splitting the duties with Carter Hutton.
Hutton won his first six starts before going into an extended slump. He’s gone 0-7-4 in his past 11 games, with his last victory a 25-save performance in a 4-3 overtime win over San Jose on Oct. 22.
Buffalo promoted goalie Jonas Johansson from Rochester, its American Hockey League affiliate.
Johansson, who has yet to appear in an NHL game, has a 13-3-3 record this season. He ranks third in the AHL with a 2.19 goals-against average and fifth with a 0.925 save percentage.
The Sabres are in the midst of playing nine of 10 games at home and host Montreal on Thursday.