Zambrano matched his career high with seven walks in his first start since a June 25 dugout blowup and ensuing banishment, and short-handed Chicago lost to the Giants 4-3 in 11 innings Monday night long after he left the game.
Pat Burrell hit a game-ending sacrifice fly off Marcos Mateo (0-1), who made his major league debut after being called up earlier in the day.
"I've been confident in Pat ever since he got here," said Aubrey Huff, who had a key single in the rally. "He has that playoff mentality. He's won a World Series."
Chris Ray (5-0) recorded the final out of the 11th for the win.
Burrell had his first game-ending RBI of the season and the 10th of his career. It was his first ever via sacrifice fly.
The Cubs lost their fourth straight and season-high sixth in a row on the road. They last dropped six straight away from Wrigley Field from June 10-25, 2009.
Zambrano returned to the rotation for the first time in six weeks since exploding in the dugout and getting into it with teammate Derrek Lee. Zambrano made three relief appearances since coming off the restricted list late last month after undergoing anger management therapy during his month away.
"Today was one of those days when I had too much movement and velocity. It was hard to control," Zambrano said. "I'd throw it down the middle and it would drop down. I'd throw to the corner and it would go wide. The good thing was I was able to throw five innings. I felt good. I've been working hard for this moment."
Posey hit an RBI double in the fifth for the Giants, who also pushed across another run on a wild pitch by Zambrano.
"We did what we wanted and created some opportunities," manager Bruce Bochy said. "You hope eventually you'll cash in, and we did."
Zambrano was in position to win after Tyler Colvin gave the Cubs a 3-2 lead on a sixth-inning single, but the Giants tied it in the bottom half on an RBI single by Torres.
Zambrano was suspended June 26, a day after he got into a verbal altercation with Lee. The two exchanged words in the dugout after Zambrano was pulled from his start after allowing four runs in the first inning against the Chicago White Sox.
He was on a pitch count of 75-80 and already had 69 pitches through four innings. Zambrano wound up at 95 pitches, only 49 strikes.
Cubs manager Lou Piniella missed the game to be in Tampa, Fla., with his ailing mother. There is no timetable for when Piniella might rejoin the club.
In addition, Lee left the team to travel to his native Sacramento to be with his ill grandfather. The Cubs placed him on the bereavement list after the game, meaning the soonest he can play again is Friday.
Scouts were out in force at AT&T Park to see Zambrano's return and evaluate whether he could help a contender if the Cubs decide to let him go. He hit 90 mph on the radar gun only a couple of times, regularly throwing 86-88.
Zambrano yelled at himself when he escaped a second-inning jam in which he walked three before striking out leadoff man Torres to get out of it unscathed.
Colvin's two-out single chased Giants starter Madison Bumgarner.
He had lost his previous two starts following a four-start winning streak, but Bumgarner didn't get a single run of support in 10 innings over those past two outings.
Chicago catcher Geovany Soto was a late lineup scratch by the Cubs because of a sprained right shoulder ligament, and he was placed on the 15-day DL after the game after missing his third straight game. Soto tested his shoulder, which bothers him only at the plate, during early batting practice and was fine but during later warmups experienced some discomfort.
Koyie Hill replaced Soto.
San Francisco earned its 28th comeback win, matching the club's 2009 total. ... Giants INF/OF Eugenio Velez began full baseball activities for the first time since being struck on the head July 25 by a foul ball in the dugout against Arizona, and is scheduled to begin playing in rehab games later this week. ... Giants LHP reliever Jeremy Affeldt, on the DL since July 24 with a strained left oblique muscle, will throw the third bullpen of his rehab on Tuesday. Affeldt will mix in curveballs this time. "I will know tomorrow where I'm at," he said. ... The Giants celebrated "Jerry Garcia Tribute Night" with a concert by a Grateful Dead tribute band before the game. Members of the actual band sang the national anthem. Former Dead drummer Mickey Hart led fans in "Take Me Out to the Ballgame" played on kazoos during the seventh-inning stretch, trying for a place in the Guinness Book of World Records.