Bernie Miklasz reports on his forum that Chris Duncan has been optioned to AAA-Memphis and Joe Mather has been recalled to replace him.
According to Bernie,
"Will be made official this afternoon...
The way it is was explained to me (no names, but the person who told me is, as we say, golden): they think Chris can be a huge asset if he gets his swing right, and gets his power back... and they really believe the best way for that to happen is for him to get out of here for a while (he's pressing) and get a chance to take 4-5 at-bats, every day."
Mather has been hitting .315/.406/.671 for the Redbirds, with 12 home runs and 24 RBI. He's especially been crushing left handed pitching, posting a 1.217 OPS against southpaws thus far in 48 AB.
Edit: Forgot the Miklasz link
And Edit #2: Here's what Joe Strauss has to say:
"The Cardinals on Friday officially reversed field by optioning left fielder Chris Duncan to Triple-A Memphis and promoting prospect Joe Mather to take his place.
The move follows Duncan’s prolonged struggle to regain the power stroke apparently sapped by last season’s sports hernia that required September surgery and a prolonged recovery."
Edit 3 is it's not breaking anymore, since the kid is playing today.