He went 5 1/3 innings, allowing just three hits, but walked four, which led to three earned runs.
Hirsh (1-1) left in the bottom of the sixth inning with the bases loaded.
After reliever Chad Qualls allowed a run, he ended the threat, leading to the preservation of Hirsh's victory.
Hirsh struck out five and lowered his earned-run average to 6.75.
On Saturday, in his debut, he'd allowed four runs — all of them on homers — in four innings.
At one point, Hirsh, who was taken in the second round of the 2003 draft by the Astros, retired eight straight Brewers before Milwaukee (57-64) tied it at 2 in the fourth.
The win ended a five-game skid for the Astros (58-63) that began with Hirsh's debut on Saturday.
The Astros are in the midst of an 11-game roadtrip that will likely see Hirsh get his next start on Tuesday against National League Central rival Cincinnati.
Freddy Sanchez (Glendale Community College, 1998) third baseman, Pittsburgh Pirates — Sanchez, playing far and away his best Major League season in his short career, continues to lead the National League in batting with a .346 average heading into today.
Since being selected to the All-Star game, Sanchez has struggled in the second half of the season. He's just recently turned it around, however.
Pittsburgh Manager Jim Tracy gave Sanchez two consecutive days off on Saturday Aug. 5 and Sunday Aug. 6, as Sanchez had gone eight for 37 in the team's last nine games, dropping his batting average from .365 on July 18 to .339 after Aug. 5.
"I'm just giving him a day off," Tracy told MLB.com. "I think it's very wise with what I've been seeing here. ... The opportunity for tomorrow [off] also will do him a world of good for beginning on Tuesday, when we start with the Houston Astros."
Since Aug. 6, the former Vaquero standout has been on quite a tear.
He's gone 14 for 32 over the last nine games, hitting at a .440 clip.
He's currently nine points ahead of the Florida Marlins' Miguel Cabrera (.337) in the race for the batting title with the Atlanta Braves' Chipper Jones (.328) in third.