Garza (2-5) is now winless in his last seven starts. The right-hander struck out the side in the first, and was cruising as the Cubs opened a 4-1 lead. Then came the Tigers sixth.
Boesch singled to lead off the inning and Miguel Cabrera reached on an error by third baseman Joe Mather, who booted a potential double play ball. One out later, Delmon Young hit a RBI single, and Peralta followed with his double, tying the game at 4.
"I just missed it," Mather said of Cabrera's ball. "I think it was the turning point in the game. Garza got out of trouble right there -- he was in a little bit of trouble and he got out of it. I feel bad that's what started it all. I feel, we get out of that inning with him still on the mound, we win this game."
"It was too bad," Cubs manager Dale Sveum said, "because [Garza] obviously had good stuff and was pretty much cruising through that game. He got a double-play ball that changed the game around. It would've been two outs, nobody on instead of 15 pitches later, they got the lead.
"It's too bad because he had command of his fastball and everything," Sveum said of the right-hander. "That was just an unfortunate outing."
The sixth wasn't over as Ramon Santiago hit a comebacker that knocked Garza backward on the mound. He said his "life flashed before his eyes," but he was able to recover and throw out Santiago at first. Peralta scored the go-ahead run on the play.
"I just saw it coming, so I thought, it's either the chest, face or stop it with my hands," Garza said of Santiago's ball. "I stopped it with my hand. I knew the guy on third was taking off on contact and I had to get an out. I just flipped it over to [Bryan] LaHair and I'm glad I got the out."
Boesch connected against Manuel Corpas for his seventh home run with one out in the seventh, and the Tigers added two more in the eighth.
Garza tried to downplay the error.
"The worst thing you can do is get [ticked] off and the next thing you know, you leave four pitches over the middle of the plate and it's a way different ballgame," he said. "I felt I made my pitches and they got enough on them to get it to where we weren't."
Peralta's hard-hit ball made up for an error in the series opener Tuesday which led to the Cubs' game-winning run.
"He put it behind him," Boesch said of Peralta. "Yesterday was yesterday, and we even talked about it. Yesterday was yesterday, and he came out and had a huge game for us."
Mather hit a RBI single in the second and another run scored on Tony Campana's groundout that inning. Alfonso Soriano and Bryan LaHair hit back-to-back RBI doubles in the Chicago fifth, which opened a 4-1 lead.
But it wasn't enough. The Cubs had scored two or fewer runs in Garza's last six outings. Unfortunately for Garza, he's used to little offensive support, saying it happened to him three years ago with the Rays.
"If you press, it just gets worse," Garza said. "For a guy like me, it's just stay in the moment, keep going and get to the next one. It's tough, not only on guys like me, but it's tough on the young guys, and not even the young guys but guys with not a lot of time here.
"If we start panicking as veterans, what does that show to the young guys?" he said. "You just come out tomorrow and get ready for the next one."
The right-hander, who is drawing interest from teams heading toward the Trade Deadline, gave up seven hits and struck out five in front of 41,326, the largest crowd of the season at Wrigley Field. Garza has been the subject of trade rumors before.
"I just want to play," he said. "It's not my choice. If they want to keep me here, I'm going to keep playing and go out there every five days and do what I'm set to do and that's get as many outs as possible and that's keep my team in the ballgame. I love the city, my kids love the city, and it would be a great place for them to grow up.
"When they tell me I can't play anymore, that'll be the worst day of my life," Garza said. "I just want to keep playing. I'll pitch on the fricking moon. As long as people keep asking me, I've done a good job. When they stop asking, it's time to worry."