Williams didn't practice Wednesday due to a back issue, Chris Brown of the Bills' official site reports.
Williams hasn't missed a game this season and wasn't reported to have suffered an injury in the Week 15 win over the Lions, so it's possible he's just getting a maintenance day as the Bills kick off their preparations for Week 16. Through his 14 appearances in 2018, Williams has logged 30 tackles, five sacks and a forced fumble.