The St. Louis Cardinals ran out of arms and reasons not to promote their top pitching prospect. Michael Wacha, the popular choice to fill rotation holes all along, will start in his major league debut Thursday against the Kansas City Royals. The Cardinals lead the majors with a 33-17 record, overcoming numerous injuries with minor-league call-ups. They filled rotation spots with less celebrated rookies John Gast and Tyler Lyons earlier in the season. The 6-foot-6, 210-pound Wacha was the Cardinals' first-round draft pick and the 19th overall selection last year out of Texas A&M. He's 4-0 with a 2.05 ERA that leads the Pacific Coast League in nine starts for Triple-A Memphis, holding opponents to a .187 batting average in his first full pro season. The 21-year-old will be the eighth rookie pitcher to debut with the Cardinals this season and gets the call after logging a total of 73 innings in the minors, 52 of this year. General manager John Mozeliak had cited Wacha's lack of professional experience in the decision to promote others earlier. Cardinals rookie pitchers lead the majors with 12 wins, including five from rookie starter Shelby Miller and four from reliever Seth Maness. Lyons, who jumped from Double-A Springfield, won his debut with seven strong innings at Los Angeles. Last season, Wacha appeared at three minor league levels with an 0.86 ERA in 11 appearances with 40 strikeouts, and appeared in two games during the Texas League playoffs for Double-A Springfield. Wacha will replace Gast, who had stepped in for Jake Westbrook but lasted just six batters in his most recent start and landed on the 15-day disabled list with a shoulder injury. Gast is the fourth starter to land on the DL, along with Westbrook, Jaime Garcia and Chris Carpenter. Garcia is out for the year following shoulder surgery, Westbrook has missed nearly a month with an elbow injury and Carpenter has yet to pitch while rehabbing from chronic nerve issues. Copyright 2013 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.