Garcia wouldn't have pitched in this round again, anyway; the Cardinals' listed starters are Chris Carpenter (Game 3) and Kyle Lohse (Game 4), with Game 1 starter Adam Wainwright working an if-necessary Game 5.
So Miller will be an extra bullpen arm for the Cardinals. The Cardinals' first-round pick in the 2009 draft, Miller pitched in six games for St. Louis this season, including a start against the Reds on the season's final day, in which he threw six innings, allowing Cincinnati just one hit.
Miller turns 22 Wednesday. That's a fine birthday gift for a young pitcher, being added to the playoff roster. Perhaps he'll return the favor by throwing well in relief in this series.