Garcia strained his left rotator cuff in Game 2 of the Cardinals' National League Division Series game against Washington and could miss the rest of the playoffs. At the very least, major league rules require he sit out at least the NL Championship Series, leaving open his possible return for the World Series should St. Louis make it that deep into the post-season.
Garcia allowed a run on two hits and three walks to the Nationals in two innings before leaving with a sore shoulder.
The 26-year-old left-hander spent more than two months on the disabled list with a sore shoulder before returning in mid-August. He was 7-7 with a 3.92 earned run average in 20 starts this season.
Miller, a right-hander who turns 22 Wednesday, had one win in six games for the Cardinals after being recalled in September when he made his first big league start. He had a 1.32 ERA with 16 strikeouts in 13 2/3 innings.
In the Cardinals' final regular-season game, Miller threw six innings of one-hit ball against the Reds.