Bell was hit by a teammate during a non-contact drill. It appeared that Bell thought the play was finished when he was taken down.
Tomlin said Bell will be evaluated later.
Bell didn't take another snap in practice and had his left leg heavily wrapped. He never left the field and took the wrap off when practice ended.
Bell is listed as a co-starter on the depth chart with Isaac Redman. He missed the team's preseason opener last week with a left knee injury.
Redman suffered a stinger in Thursday's practice, according to Tomlin.
The Steelers play at Washington on Monday.