The extra man on the roster for Tuesday's doubleheader, second baseman Scooter Gennett, will stay up to fill Gallardo's roster spot.
Leaving Tuesday's start in the fifth inning, Gallardo likely aggravated a nagging hamstring when he dove to catch a bunt attempt in the bottom of the fourth inning.
Gallardo stayed in, hit in the top of the fifth inning and returned to the mound for the bottom of the frame. He was forced to leave the game while pitching to Cubs second baseman Darwin Barney with the bases loaded and two outs.
In the midst of the worst season of his career, Gallardo admitted the hamstring has been bothering him since before the All-Star break to reporters after the game. Gallardo is 8-9 with a 4.91 ERA in 23 starts this season.
With Gallardo missing at least three starts, rookie right-hander Tyler Thornburg will likely jump into the rotation and start Monday in San Francisco. The 24-year-old tossed six scoreless innings in the second game of Tuesday's doubleheader.
Gennett was officially optioned to Nashville following the doubleheader, as Major League Baseball allows teams to carry a 26th player for a scheduled doubleheader but then requires the player to be sent down right after.
The 23-year-old started the second game Tuesday and went 1-for-4 with a strikeout. He entered Wednesday's game hitting .217 with a home run and five RBI in 18 games with the Brewers this season.
Before coming up, Gennett was in a slump at Triple-A, lowering his batting average to .280.
Rogers takes first step: A long and frustrating year for Mark Rogers began to move forward Wednesday, as the right-hander made a rehab start for Milwaukee's rookie team in Arizona.
In a game for the first time since being pulled off a rehabilitation start in May, Rogers allowed just one hit and struck out four in two innings of work.
Rogers' right shoulder has bothered him from spring training, as the injury-plagued former first-round draft pick battled lost velocity and shaky command. The 27-year-old made five rehab appearances for Double-A Huntsville, but had an ERA of 11.12 and was pulled off the assignment on May 9.
Rogers posted a 3.92 ERA in seven starts late in the season for the Brewers in 2012 and was expected to compete for a spot in the rotation or bullpen during spring training.
Another first baseman: Jonathan Lucroy made his first career start at first base Wednesday, becoming Milwaukee's 12th different starting first baseman since Prince Fielder left after the 2011 season.
Lucroy is the seventh different starting first baseman for the Brewers this season, joining Yuniesky Betancourt, Juan Francisco, Alex Gonzalez, Martin Maldonado, Sean Halton and Blake Lalli.