A's designated hitter Mike Piazza is expected to be sidelined 4-to-6 weeks with a strained right shoulder injured Wednesday night when he dove into third base in an unsuccessful attempt to avoid being tagged out. With two outs in the sixth, Bobby Crosby hit a grounder to Red Sox third baseman Mike Lowell. Piazza, who was on second, and Lowell both charged for the bag and collided near it. Piazza was tagged out and stayed on the ground in pain for a short time. He thought the shoulder was separated, but the injury was diagnosed as a strained AC joint.