Brewers announced Wednesday that the top pitching prospect will return to Milwaukee and will join the club for Friday's game against Pittsburgh.
To clear room for Thornburg, right-hander Tim Dillard was outrighted to Nashville, pending his approval.
Brought up from Double-A Huntsville to make an emergency start against Toronto on June 19, the 23-year-old Thornburg was impressive for five innings but ran out of gas, allowing three straight home runs in the sixth.
He was sent back to Nashville immediately after but went on the disabled list with a sore right wrist. Thornburg was activated and made his first Triple-A start July 4, allowing two runs in five innings and earning the victory.
The move leaves the Brewers with an abundance of starting pitchers on their active roster. With Yovani Gallardo, Zack Greinke, Randy Wolf, Mike Fiers and Marco Estrada currently making up the rotation, Thornburg makes six. Injured starter Shaun Marcum is due back soon and would be the seventh starter.
Dillard, 28, has pitched in parts of four seasons with the Brewers, bouncing back and forth between Nashville and Milwaukee since 2008.
This season, he has posted a 4.38 ERA in 37 innings out of the bullpen for Milwaukee.
In another transaction Wednesday, the Brewers announced the addition of infielder Jeff Bianchi to their roster from Triple-A.
The 25-year-old Bianchi was claimed off waivers from the Cubs in January and was a second-round pick of Kansas City in 2005.
Bianchi has spent time in Double-A and Triple-A in 2012, hitting .351 in Huntsville and .305 in Nashville. His first appearance with the Brewers will mark his major league debut.