The move isn't surprising, considering Roethlisberger did not practice much of the short week leading up to the game. His four turnovers last week in San Francisco were a large reason why the Steelers lost.
The Rams have one of the worst defenses in the game, and currently rank last in run defense. It would be logical for the Steelers to feature the run in this short-week game.
RB Rashard Mendenhall, who's nursing bruised ribs, hasn't had more than 18 carries in a game since Week 5, when he had 23 rushes for 146 yards and a touchdown. He was named AFC Offensive Player of the Week for his performance, but has been relatively quiet since then.
He's run stronger over the last few weeks, and while his numbers aren't outstanding, he's been effective. It's likely he'll still share some carries with Isaac Redman as well.
The announcement is Roethlisberger will not start; nothing has been reported on whether he will dress for the game. That could play into the Steelers' tenuous situation at the running back position. Mewelde Moore will not play in this game, leaving the Steelers with only Mendenhall and Redman. Speculation that RB John Clay may not be called up from the practice squad to fill the third running back spot exists. If Roethlisberger isn't activated for the game, Clay could be called up.
As it is, Steelers offensive coordinator Bruce Arians has said this week they may go with just two active running backs.