Ullmark exited in the first period of Thursday's 4-3 overtime loss to the New Jersey Devils and didn't come back. The 27-year-old owns a 2.44 goals-against average and a .919 save percentage over 12 games this season.
Meanwhile, defenseman Rasmus Ristolainen returned against the Philadelphia Flyers on Saturday after his grueling bout with COVID-19 earlier this month.
The Finnish blue-liner recently opened up about his struggles with significant drops in oxygen levels, intense chest pain, and fatigue. The 26-year-old hadn't played since Jan. 31.
Sabres captain Jack Eichel remains day-to-day and missed his second straight game, but he could return Sunday. Winger Jeff Skinner, who had been a healthy scratch during the team's previous three contests, also returned to the lineup Saturday.