Miller went 0-4 and allowed 13 earned runs in 22 innings against Pittsburgh. He allowed six homers, walked nine and struck out 21. Seven of those runs were scored in 10 1/3 innings at PNC Park.
"It wasn't an easy decision, because we really like what Shelby has done -- 15 wins this season, and [he has] really done a nice job with every opportunity we've given him," Matheny said. "He's adapted and is a guy who can make the changes that are necessary for him to have success against a team that's starting to get on him."
Miller will be available to pitch out of the bullpen for the remainder of the series -- he allowed a solo home run to Starling Marte to go with a strikeout of Justin Morneau during the eighth inning of Game 2 -- and the Cards haven't ruled out giving the rookie right-hander a start if they advance deeper into the postseason.
As for Wacha, a start on Monday will be just the latest new experience in a season of firsts for the rookie. A first-round pick out of Texas A&M in the 2012 First-Year Player Draft, Wacha, 22, is less than five months removed from making his Major League debut.
"It's a great opportunity," Wacha said. "I wish we could have won today, and maybe I wouldn't have had to pitch. But I just look forward to the opportunity of getting out there and pitching in Pittsburgh."
Wacha made two appearances -- one as a starter, one out of the bullpen -- against the Pirates this season and shut Pittsburgh out on two hits and two walks in nine combined innings. After being inserted back into the rotation for the season's final month, Wacha boasted a 1.72 ERA in five September starts.
This will be just career road start No. 4 for Wacha and his first at PNC Park, where the crowd is expected to be raucous. The game is also guaranteed to be an elimination one for either the Pirates or the Cardinals.
"You can't get too caught up in the moment," Wacha said. "That's whenever I get in trouble -- getting too caught up in the moment, leaving pitches up, letting the emotions get the best of me. If I can slow the game down, control the counts, I think it will work in my favor."
In his last outing -- a Sept. 24 start against the Nationals -- Wacha carried a no-hitter through 8 2/3 innings. It was broken up by an infield single.
"He just doesn't look like a kid that is fresh out of college," Matheny said of Wacha. "The stuff is there, but there is just a composure, there's a maturity, a pitching maturity. The situation doesn't really get much bigger than it was his last start, and you watched him be able to hold that and continue all the way through his last pitch. It was something that was impressive for us to watch and continue to watch him grow. There's not a whole lot that's there that you wouldn't like."
Wacha threw a bullpen session at Busch Stadium on Friday morning in preparation for Monday's start.