The Steelers officially ruled the veteran running back out of Saturday's wild-card game on Friday after Williams missed his third straight practice with an injured right foot.
Fitzgerald Toussaint and Jordan Todman will fill in for Williams, who may be available later in the playoffs if Pittsburgh advances. The 32-year-old tied for the NFL lead with 11 rushing touchdowns this season after taking over for All-Pro Le'Veon Bell, who is out for the season with a right knee injury.
Toussaint ran for 24 yards and caught a 2-point conversion after Williams went down in the regular season finale last week against Cleveland. Todman ran for 22 yards on four carries in limited action this season.
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