The calf injury sustained by Jerome Bettis is not serious, but he will miss at least the next two weeks and leave the Steelers without their top two running backs for the start of the regular season.

That means Willie Parker, a second-year running back from North Carolina, is expected to start the season opener Sept. 11 against the Tennessee Titans.

Bettis had a magnetic resonance imaging Saturday, a day after his calf was injured in the first quarter of a 17-10 preseason loss to the Washington Redskins, but the results were inconclusive because Bettis had too much blood in his calf. It is possible he could have another MRI early this week.

The Steelers are already without Duce Staley, who has missed the entire preseason and probably won't return until at least Week 3 against the New England Patriots, and maybe longer. Coach Bill Cowher said last week he doesn't want to rush Staley back into the lineup, but he said that before Bettis was injured.

Parker has become the big-play back, registering runs of 50 yards or longer in each of his past two preseason games. After a 51-yard run in the first quarter against the Redskins -- his only carry of the game -- Parker was removed from the game because Cowher didn't want to risk injury to another of his running backs.

Cowher said he might even consider not using Parker in Thursday night's preseason finale in Carolina. The Steelers have only two other halfbacks on the roster -- third-down back Verron Haynes and rookie Noah Herron.