Coach Doug Marrone provided few details regarding the nature of the injury. He wouldn't say which foot Williams had hurt or how and when it happened.
''Mario reported to our athletic training staff that he had a sore foot,'' Marrone said, following the two-plus hour session at the team's training camp facility in suburban Rochester. ''So what we wanted to do was hold him out of practice and let our medical staff evaluate him.''
Marrone couldn't say how much time Williams might miss because the injury is still being evaluated. The Bills are back on the field Monday afternoon.
Williams declined comment after practice, and deferred all questions to Marrone.
After acknowledging he was disappointed to miss the opening session, Williams then apologized that he couldn't say anything more.
Williams did not appear to be walking with a limp. He did not participate in any individual or team drills, and spent much of practice watching from the sideline with his helmet on. On several occasions, he was spotted bending over frontward to stretch out his back.
The foot injury came as a surprise after Williams did not indicate he was experiencing any health issues upon reporting to camp on Friday.
Williams was motivated upon arriving at camp, saying he wanted to put behind his inconsistent first season in Buffalo, which came after he signed a six-year, $100 million contract with the team in free agency. Williams led the Bills with 10.5 sacks, but his performance was hampered in the first half of the season because of a left wrist injury that was surgically repaired in late October.
Starting defensive tackle Kyle Williams was limited in practice, as expected. The team intends to ease the player back in after Williams had surgery this offseason to remove bone spurs from his right foot.