Coach Chan Gailey provided few details Thursday except to say Kelsay was undergoing tests, and he'd know more on Friday. The 10th-year veteran was escorted off the field by a team trainer shortly after the start of practice.
The Bills (3-5) are already depleted at defensive end in preparing to play the Patriots (5-3) on Sunday. Starter Mark Anderson will miss his fourth game with a left knee injury.
Backup defensive end Shawne Merriman was also added to Buffalo's injury report with a chest injury.
Receiver Stevie Johnson (bruised thigh) and defensive tackle Kyle Williams (ankle) returned to practice, and both are expected to play.