"Right now, it's week-to-week, and we don't have many weeks left," Granato said. "So, that one's a little touchy."
The Sabres have 13 games left, with their regular season scheduled to conclude May 8.
Ullmark previously missed 15 games this season due to injury. He's been one of few bright spots for Buffalo in 2021, posting a .917 save percentage across 20 contests.
The 27-year-old is an unrestricted free agent at season's end.