Williams said he injured his back getting out of bed Friday morning.
"I guess I slept on it wrong. I sat up in bed, and when I stepped up to get out, it just kind of hitched up on me," he said. "I guess I just slept on it funny and then jumped up too fast."
Williams, who has battled a lingering Achilles' injury this season, said this was the first time he has suffered a back spasm. His status for Sunday remains unclear.
"We've got basically two days. We'll see when we get to Sunday. I don't know how to gauge it right now. We're working, definitely looking to play," he said. "I plan on playing, but if you can't, you're only going to hurt everybody else."
The Bills' longest-tenured player is coming off his best game of the season, when he had two sacks and forced a fumble in a 37-14 win over the New York Jets.
Williams hasn't missed a game this season, a streak he hopes to continue.
"Lot of times when it's getting better, it will go away just as quick as it came," he said. "That's kind of what we're hanging our hat on."
Friday's full injury report:
DT Kyle Williams (back; did not participate)
WR Stevie Johnson (groin; full participation)
WR Robert Woods (ankle; full participation)
CB Nickell Robey (ankle; full participation)