The Chicago Bears have their clear starter at running back in Matt Forte, and brought in Michael Bush this offseason to be his backup. On the season, Bush has carried the ball 114 times for 411 yards with five touchdowns. He only has a 3.6 yards per carry average, but he's essentially exactly what you look for in a backup.
On Sunday, he didn't get any carries behind Forte, though. Forte carried the ball 20 times for 69 yards with no touchdowns, quarterback Jay Cutler had a 9-yard scramble and ... Armando Allen received two carries in the reserve role.
Allen is the third running back on the roster, and was Chicago's fourth choice behind Lorenzo Booker, who was placed on injured reserve earlier in the season. Of course, two carries isn't much at all, but the fact that he came off the bench while Bush remained behind is something of an oddity.
As noted by the Chicago Tribune, Bush was active and able to play on Sunday. He said that he was dressed, in uniform, didn't have broken ribs and that he was totally able to play, so it was a decision that came from the coaching staff.