Mendenhall missed Pittsburgh's first four games of the 2012 season after tearing his ACL in the final game of the 2011 season. He returned in Week 5, and played well. But the running back left Pittsburgh's Week 6 game on Thursday night with an Achilles injury.
The injury was not originally believed to be severe. Especially given the extra few days of rest, Mendenhall was expected to recover in time for this week's game in Cincinnati.
Since backup Isaac Redman is also unlikely to play (he, too, missed practice on Friday), Jonathan Dwyer could be the replacement starter. Dwyer had a very productive college career at Georgia Tech, but has not yet emerged as a play-maker for the Steelers. The third-year back has carried the ball 24 times for just 70 yards this season.