The Brewers' bullpen quickly surrendered a two-run lead that had once seemed so secure in a 3-2 loss to the St. Louis Cardinals on Sunday.
"It was bad enough where I had to take myself out of the game and I don't do that," Garza said. "There's too much left in the season and I thought I'd make the smart play."
Manager Ron Roenicke said Garza will be examined on Monday after feeling his side grab on one of his final pitches.
"He couldn't keep going," Roenicke said. "Great game, great fastball. He had a lot working."
Garza said he had a similar injury last spring.
"I hadn't even thought of it and it just happened, and you try and think of what you did," Garza said. "It felt like a muscle grab, and it was one of those, and son of a gun."
The Cardinals took two of three from NL Central-leading Milwaukee and pulled within one game of the division lead.
"We played a good ballgame," Roenicke said. "Got a couple of runs of Lackey and put up some zeroes and you feel pretty good about the game.
"It didn't work out."
Lackey (1-0) was the Cardinals' big catch at the trade deadline that capitalized on Boston's sell-off. The right-hander won the clinching Game 6 of the World Series against St. Louis last fall allowed two runs and seven hits in seven innings and is 12-7 overall.
Lyle Overbay pinch-hit for Garza in the seventh with two outs, a runner on first and the Brewers up 2-0. Garza had been coming up the dugout steps, but Roenicke said the pitcher hadn't heard him say he was coming out.
"When you're talking about obliques, you can't go out there," Roenicke said.
Overbay singled, raising his career average to .364 against Lackey, but Gomez popped up. Garza was out after just 71 pitches, and the Brewers' bullpen quickly surrendered the lead.
Garza needed just 38 pitches and faced the minimum the first four innings. He went to a full count for the first time against Matt Adams, who doubled leading off the fifth.
Cardinals: Outfielder Jon Jay missed his fifth straight start with a sore wrist from a checked swing in San Diego. He had to be extra disappointed, given he's 8 for 13 with a homer against Garza.
Cardinals: After an off-day, the Cardinals host Boston for a three-game series beginning Tuesday night. On Wednesday, Shelby Miller (8-8, 4.14) faces ex-teammate and close friend Joe Kelly (2-2, 4.37), who was traded to Boston in the Lackey trade. Miller and Kelly were the best man in each other's wedding.
Ramirez is batting .522 (12 for 23) during a six-game hitting streak. He's batting .459 (17 for 37) against the Cardinals this year with two homers and nine RBIs. He has 37 homers and 139 RBIs against St. Louis, most among active players.