Rookie receiver Terrance Williams left practice Friday after landing hard on his back and hitting the back of his head.
He was in a coverage drill against cornerback Brandon Underwood. They went up for a pass, and Williams was pushed to the ground. He got up slowly and went to the sideline, where trainers examined him. He stood, helmet off, and appeared to be given a brief concussion test.
He left the field with a trainer and did not come back to practice.
A concussion might cost Williams two preseason games, at least. The Cowboys play Miami on Sunday and Oakland five days later.
-- Carlos Mendez