Williams, 27, was the lone player left on the Bills’ COVID-19 list.
Williams appeared in four games for the Bills last season, catching 12 passes for 166 yards and a touchdown. He joined the team after leading the CFL in receiving yards (1,579) in 2018 with Edmonton.
Buffalo signed King a week ago after E.J. Gaines opted out of playing the 2020 season.
King made 38 tackles while appearing in 29 games for the Seahawks the past two seasons. He opened his career as a 2015 seventh-round pick in Atlanta and played five games during his time with the Falcons.