Combined, Flaherty has struck out 147 and walked 35 in 148 2/3 innings. Flaherty, a first-round Draft pick out of Harvard-Westlake High School in California in 2014, represented the Cardinals in the All-Star Futures Game, and he earns this big league summons before his 22nd birthday.
"I've been anxious," manager Mike Matheny said of getting Flaherty to St. Louis. "I've been following him since his Draft and watching him all along and watching the kind of season that he's had. He's shown us that he's one of the top prospects we have in the organization, and when we're trying to make a strong push here, we're going to give an opportunity to let these kids do their thing."
Flaherty's addition brings the average age of the Cardinals' five starters down to 25. Michael Wacha, Carlos Martinez, Luke Weaver and Flaherty are all 26 or younger.
"I still think we expect to compete, we're just going to do it with a different name and a different face," Cardinals president of baseball operations John Mozeliak said after trading Mike Leake to open a spot in the rotation. "Ultimately, this is about creating that opportunity internally. We certainly feel like we're still in this and expect to have success."
Flaherty will have to be added to the 40-man roster on Friday, and he will arrive along with several other September callups. How many starts he'll be able to make will depend upon how comfortable the Cardinals are in pushing his innings total higher. He's already pushed past his previous season-high of 134 innings, and the Cardinals had intended to build in only a 10-20 percent innings increase from that.
"We're going to keep an eye on him," Matheny said. "As we get into this first one, let's get a look at him, see how it goes and decide from there."
Jenifer Langosch has covered the Cardinals for MLB.com since 2012, and previously covered the Pirates from 2007-11. Follow her on Twitter and Facebook. This story was not subject to the approval of Major League Baseball or its clubs.