Single-A Palm Beach placed Showalter on its 7-day injured list Aug. 16 with an unspecified injury.
Showalter, 20, posted a 2.62 ERA and 1.26 WHIP with 59 strikeouts over 34.1 innings before being shut down with the injury. The Cardinals appear committed to developing him as a reliever, as he had made each of his 24 appearances out of the bullpen this season.