Casey was at par through six holes when he withdrew Thursday in the first round. He says he first injured the rib muscle a week before the British Open.
Asked if he would play next week at Hazeltine, Casey says he must be cautious because he has a lot of tournaments left this season. He mentioned the FedEx Cup on the PGA Tour and Race to Dubai in Europe.
Casey says he will stay in Ohio for treatment to see if the injury improves.