"After 15 years of playing ball, my mind was still in it but my body wasn't," Zimmermann said, according to MLB.com's Adam McCalvy.
Zimmermann was set to retire last month before the Brewers called him up on April 29.
The 35-year-old Wisconsin native made two appearances for his home-state Brewers, posting a 7.94 ERA and 6.99 FIP in 5 2/3 innings.
Zimmermann's career appeared to be over before he signed a minor-league deal with the Brewers in February. He was released then re-signed by Milwaukee at the end of March.
His best years came with the Washington Nationals, where he was a two-time All-Star, threw a no-hitter, and received Cy Young consideration in 2013 and 2014. Overall, he won 70 games and owned a 3.32 ERA in 178 starts for the Nats.
Zimmermann turned his success in Washington into a five-year, $110-million deal with the Detroit Tigers before the 2016 season. He pitched to a 25-41 record with a 5.63 ERA for the Tigers.