"I'm looking forward to the opportunity," Thornburg said. "I think this experience is great and it will help me settle in."
The right-hander will pitch in place of Zack Greinke, who will be skipped in the rotation to regain the sharpness he lost while starting three consecutive games around the All-Star break.
Thornburg started for Double-A Huntsville on June 9 and didn't pitch again until his MLB debut for the Brewers against the Blue Jays on June 19. The Brewers optioned him to Triple-A Nashville after that outing, and Thornburg didn't make his Sounds debut until July 4 because of a minor right wrist injury.
After being recalled over the All-Star break to pitch in relief, Thornburg allowed a solo home run in two innings against the Pirates on Saturday.
"It's a little different than I was expecting, I'm not gonna lie," Thornburg said of his bizarre rookie season. "You never know what's going to come next, I guess."
Meanwhile, seasoned veteran Adam Wainwright will get the nod for the Cardinals, ready to secure the win in the rubber match of the three-game set.
On Friday against the Reds, the right-hander took the loss, surrendering four runs on five hits in 6 1/3 innings of work. Striking out four and walking two, Wainwright fell to 7-9 on the season with a 4.62 ERA.
However, he's dominated the Brewers since 2009 with a 1.61 ERA, the lowest ERA against Milwaukee among Major League pitchers.
"His leadership's important and how well he's been throwing the ball," St. Louis manager Mike Matheny said. "Every fifth day, whoever that guy is on the mound is our most important guy, in my opinion. He's done a nice job taking advantage of the opportunities when his starts come, and then in between he's doing everything he can to help out on the other side."
Cardinals: Holliday leaves Tuesday's game
Left fielder Matt Holliday left Tuesday's game against the Brewers with a left thigh contusion after taking a Randy Wolf pitch to the leg in the top of the first inning.
Slow to get up, Holliday was looked at by a trainer before slowly making his way to first base. He played the bottom half of the inning in left field, but was replaced by Jon Jay in the second inning.
Holliday entered the game on a 15-game hit streak, batting .390 with 12 RBIs and two home runs in that span. Despite leaving the game, the streak is still alive since he did not register an official at-bat.
Brewers: Axford out as team's closer
After blowing Monday's save against the Cardinals, John Axford was told by manager Ron Roenicke on Tuesday that Francisco Rodriguez would take over ninth-inning duties for the foreseeable future.
"It was a decision I was kind of expecting after [Monday's] performance," Axford said. "It's frustrating the way things have been going this year, so maybe it will be a breath of fresh air to step back a little bit and try to get back into the swing of things, the way I was last year."
Axford converted 46 of 48 save chances in 2011, the best-ever season for a Brewers closer, but is 16-for-22 this year with a 5.35 ERA and 21 walks in 37 innings -- vs. 25 walks in 73 2/3 innings in 2011.
Rodriguez got the save in Tuesday's game, the 293rd of his career.
Left fielder Ryan Braun exited early on Tuesday with a strained right groin. Braun appeared to pull up while fielding Rafael Furcal's base hit in the fifth inning, but remained in the game until the seventh, when he was replaced in left field as part of a double switch.
On Monday, the Brewers Community Foundation passed the $1 million mark in gross 2012 revenue from its nightly "50/50" raffle. Half of the proceeds fund grants to nonprofit organizations that provide programming focusing on health, education, recreation and basic needs. Last year, the program didn't reach $1 million until Aug. 18.
As part of the 2012 Hall of Fame Awards presentation on Saturday in Cooperstown, N.Y., three World Series-winning Cardinals managers will be honored. The presentation starts at 4:30 p.m. ET at Doubleday Field and will feature Red Schoendienst (1967 World Series), Whitey Herzog (1982) and Tony La Russa (2006 and 2011). The 2011 World Series Trophy also will make an appearance.