Jackson suffered a quadriceps injury during the Rams' season-opening loss against Philadelphia and sat out last week's game against the New York Giants.
Williams spelled Jackson after his injury and started last week, rushing for a combined 127 yards on 32 carries. He is dealing with a hamstring injury.
Wide receiver Danny Amendola is listed as doubtful for Sunday's game with a dislocated left elbow. Amendola, the Rams' leading receiver last year, suffered his injury early in the fourth quarter against the Eagles.