That's because the Bills placed Williams on Injured Reserve with a designation to return on Wednesday. He lands there because of the same neck injury that landed him in the hospital during Week 2, which cost him two additional games after that, and which limited him during last Sunday's win in Tennessee. Williams played 60 snaps in that win, which makes his landing on IR all the more unusual.
Buffalo signed Josh Bush to take Williams' place on the 53-man roster; Bush spent time with Rex Ryan as a member of the New York Jets from 2012-14, and was most recently with the Denver Broncos. When he's healthy, Bacarri Rambo will assume Williams' spot in the starting lineup, with Duke Williams working in as the third safety. If Rambo can't play in Week 6, Duke Williams will start against a dangerous Cincinnati Bengals passing attack.
Williams joins quarterback Tyrod Taylor and running backs LeSean McCoy and Karlos Williams as Bills players that have finished games this season despite dealing with injuries that could cost each multiple games. That's sort of weird, right?