General manager John Mozeliak confirmed the transaction on Tuesday morning. Later in the day, the club announced that infielder Brian Barden has been recalled from Triple-A Memphis to take Duncan's roster spot.
Duncan, 28, underwent a rare surgical procedure over the winter, having a titanium disk inserted in his neck. He started the season well, hitting .304 and slugging .522 in April. Since then, though, he has steadily trended downward. In May, Duncan hit .227 and slugged .386, and in June his slugging percentage fell all the way to .289. Duncan is 1-for-31 since June 29.
On Tuesday, manager Tony La Russa angrily criticized portions of the St. Louis fan base and media who, in his opinion, have singled out Duncan unfairly as opposed to other Cardinals players who have slumped at the plate this year.
Matthew Leach is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.