The team announced Friday that it ruled out Bell, who suffered a hyperextended knee in Week 17.
Bell told NFL Media's Aditi Kinkhabwala on Thursday that he didn't attempt to run because he didn't feel comfortable testing the knee.
Coach Mike Tomlin told reporters earlier in the week that his team was preparing to play as if the best dual-threat running back in the NFL wouldn't be able to play.
Harris flashed some speed on a 59-yard scamper in Week 17 that was ultimately called back due to a penalty. However, he has just 16 yards on nine carries this season. Bell, in particular, displayed confidence this week that the rookie could handle the load if needed.