Duncan first sustained the injury in the Cardinals' season opener on Tuesday against Colorado, but he finished the game. He was kept out of Wednesday night's game and was considered completely unavailable. On Thursday, he again did not start, but he took some batting practice and could be available in an emergency.
"He's taking swings, so that's an improvement," manager Tony La Russa said on Thursday morning. "We'll see how he comes out of the swings and then talk to [head athletic trainer] Barry [Weinberg] about it. Obviously we're going to try not to use him."
With left-handers starting for Washington each of the first two games of the Cardinals' weekend series against the Nationals, Duncan might not appear in the lineup even if he were healthy. Given his hamstring condition, it's likely that he will next start on Sunday against right-hander Tim Redding.
Matthew Leach is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.