Meaning of Bell sitting out Saturday

Earlier this week, Pittsburgh Steelers coach Mike Tomlin said that Le'Veon Bell would suit up and take some snaps with the first team in their preseason opener against the New York Giants Saturday. However, Bell ended up being scratched with a sore knee. That left LaRod Stephens-Howling leading the way with 7 rushes for 40 yards, Jonathan Dwyer with 6 rushes for 15 yards, Baron Batch with 5 rushes for 4 yards, and Isaac Redman with 2 rushes for 7 yards; none of them crossed the goal line with ball in hand.