KANSAS CITY, Mo. -- The St. Louis Cardinals ran out of reasons not to promote their top pitching prospect.
Michael Wacha, the Cardinals' first-round pick last season, will start in his major league debut Thursday against the Kansas City Royals after a slew of injuries to their starting rotation left general manager John Mozeliak virtually out of options.
The 21-year-old Wacha dazzled during spring training and has been just as good at Triple-A Memphis, going 4-0 in nine starts with a 2.05 ERA that leads the Pacific Coast League.
"We just have to manage expectations a little bit," Mozeliak said before Tuesday night's game in Kansas City. "He's a year removed from college and he's thrust into a very competitive division with a lot of expectations being put on him, and I don't know how fair that's going to be.
"In an ideal world, we wouldn't have to go down this path," Mozeliak said. "Due to all the injuries, we do need some help, and he has certainly been our best pitcher at Triple-A."
Wacha, the 19th overall pick out of Texas A&M, will be the eighth rookie pitcher to debut with the Cardinals this season, and get the call after logging just 73 innings in the minors.
"What's the expectation? For him to be himself," Cardinals manager Mike Matheny said. "That's a clear message we've shared with it seems like a revolving door of young players here. Pitch your game, play the way you know how and trust the guys around you."
The Cardinals entered Tuesday night's game leading the majors at 33-17 despite four starters landing on the disabled list: Jaime Garcia is out for the year following shoulder surgery, Jake Westbrook has missed nearly a month with an elbow injury, Chris Carpenter has yet to pitch due to a chronic nerve issue, and John Gast recently went down with a shoulder injury.
Gast had been called upon to replace Westbrook and won his first two starts, but the 24-year-old lasted just six batters in his most recent start and landed on the DL.
Still, the Cardinals' batch of rookie pitchers has been keys to their success. They had already piled up 12 wins, including five from Shelby Miller, four from relief pitcher Seth Maness and one from Tyler Lyons -- the left-hander who hamstrung the Los Angeles Angels in his debut after jumping from Double-A Springfield, and who was facing the Royals on Tuesday night.
Wacha will be replacing Gast in the Cardinals' rotation.
Wacha shot through the Cardinals' minor league system last season, piling up a 0.86 ERA in 11 appearances with 40 strikeouts. He also pitched in two games during the Texas League playoffs.
"A lot's being made of this, but the fact is we've had a lot of injuries, he's someone who's pitched very well, and right now we need someone to come up here and fill in for a start," Mozeliak said. "There's a lot of questions of what this is going to look like in terms of usage -- `Will he be here for the whole season?' I think these are questions we don't have answers to at this time, but we are going to be very cognizant of pitch counts and of course innings."
Mozeliak acknowledged the Cardinals have had discussions about how much they'll use Wacha, both in his debut and this season, but he wouldn't make those thoughts public.
"We want to have the ability to adapt and adjust depending on how things go for the young man," he said. "As far as total innings for the year, someone who's coming from less than 150 a year ago, we have to be very smart about that, too."
Wacha's rapid rise to the majors isn't unprecedented, but it is unique. For reference, the Royals selected right-hander Kyle Zimmer with the fifth overall pick last summer -- 14 spots ahead of Wacha -- and he's mired at Class-A Wilmington, where he was 0-4 with a 5.28 ERA.
"A lot of it is media hype," Mozeliak said, "but understandable in the sense that he was drafted last year and not much was said of him last season, and then he had that pretty eye-opening spring for people who got to see him day-in and day-out.
"It's well deserved," he added. "He was good this spring and very good at Triple-A."