Williams, who had left the campus of St. John Fisher College to have his sore foot evaluated by doctors Tuesday, was in uniform but worked with the strength and conditioning staff on the sideline Wednesday.
Coach Doug Marrone said Williams is "day to day" and has been advised by doctors to rest his foot.
Williams declined to comment on the specifics of the injury Wednesday.
"It's not that serious," he said. "This is precaution. I'm just following orders."
The Bills have an off day Thursday before returning to practice Friday.
The Bills made a big splash in free agency in March 2012 when they signed Williams to a six-year, $100 million contract.
Williams, however, had an inconsistent first season in Buffalo. He was bothered by an injured left wrist, which he had surgically repaired during the team's bye week in late October.
He finished the season with a team-leading 10.5 sacks, seven of them coming after he had the operation.
Williams has no concerns his sore foot could eventually become as problematic as his wrist injury was last year.
"No, I'm not worried about that," he said.
Information from The Associated Press was used in this report.