What is wrong, though, remains to be seen. The right-hander is back in Milwaukee, and head team physician William Raasch reviewed Thornburg's MRI. The club is holding off on an official diagnosis until Thornburg can see a radiologist Tuesday or Wednesday.
"He doesn't want to say what it is yet," Roenicke said.
This season is Thornburg's first as a full-time reliever. In 29 2/3 innings across 27 appearances, he owns a 4.25 ERA and 1.52 WHIP. Thornburg is also striking out nearly a batter per inning but walking an average of 6.4 per nine innings, both above his career averages.
According to brooksbaseball.net, his fastball has been around 94 mph -- about 2 mph faster than it was when he started for Milwaukee last September. A small velocity jump is standard when a pitcher converts to strictly relieving.
Right-hander Mike Fiers was called up to replace Thornburg, but he has not pitched as of Tuesday.
Tim Healey is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.