''His resume speaks for itself,'' general manager John Mozeliak said. ''When he was throwing, he could be dominant. And frankly, he's very fresh.''
The 23-year-old Wacha will replace Justin Masterson on Thursday at Milwaukee. He'll be making his first start since mid-June against the Cardinals' closest competition in the NL Central.
''I knew that might have been in consideration, didn't know if they were going to do it or not,'' Wacha said. ''But I'm real happy with it. Definitely looking forward to Thursday.''
Manager Mike Matheny said Wacha, a breakout rookie star last year who was the NL championship series MVP, will be limited to around 60 pitches at Milwaukee. The 23-year-old right-hander is 5-5 with a 2.79 ERA in 15 starts and has been on the 15-day disabled list with a right shoulder stress reaction.
Mozeliak said Wacha could have made a second rehab start for Triple-A Memphis, but thought pitching in the majors ''made the most sense.''
Rookie Marco Gonzales and Tyler Lyons, lefties who were among five players added Tuesday, will be ready for long relief in that game.
''He'll have some help close behind him,'' Matheny said.
If all goes well, Wacha will likely add 15-20 pitches per start and be at full strength for the postseason.
''I feel as strong as I've ever been,'' Wacha said. ''Will definitely last me for the rest of this year, hopefully. Just sticking with the rehab stuff and this whole process. It's been a grind, happy to be out there Thursday.''
Masterson, acquired at the trade deadline from Cleveland, was 2-3 with a 7.90 ERA in six starts. He had allowed 24 earned runs in 27 1-3 innings.
The Cardinals activated reliever Jason Motte from DL from a lower back strain and also recalled lefty Nick Greenwood and catcher Tony Cruz before Tuesday's game against the Pirates.
Lyons and Gonzales were standouts in a doubleheader against the Cubs on Saturday. Lyons had a career-high eight strikeouts, the most by a Cardinals reliever since 1978, in 4 2-3 innings of relief in Game 1. Gonzales worked six strong innings in his first career victory.
Motte has been on the DL since Aug. 2. He was the closer before reconstructive elbow surgery sidelined him all last season and is 0-0 with a 4.91 ERA in 24 games this season.