The Leeds medical staff expect their early diagnosis to be confirmed after a scan today.
The 32-year-old suffered the injury after only 17 minutes in the 3-0 FA Cup defeat against Birmingham City at St Andrews in only his third start on loan from Leicester.
"Nathan's in a very bad way. We will keep him at the club for as long as we can," Kevin Blackwell said.
"He's a great lad and we enjoy having him around, but it's back to the drawing board I am afraid. I will have to see if I can come up with something else."
Blake, who left St Andrews on crutches, is expected to remain at Thorp Arch until his loan period from Leicester City expires later this month.