Molina -- batting .330 with eight home runs, 30 doubles and 54 RBI in 98 games this season -- came out of the Tuesday's doubleheader in Pittsburgh.
The catcher was sent home for tests on the bothersome knee, which has been hampering him since June. A return date will be determined after an examination.
The Cards also announced outfielder Shane Robinson was put on the DL with a right shoulder strain, while outfielders Brock Peterson and Adron Chambers were recalled from Class AAA Memphis.
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