He took the mound for his last start of the season in the nightcap, after Chicago lost Game 1. Zambrano had struggled at times this season and served two stints on the disabled list, but he threw a two-hit shutout in his last start, Friday against the Giants.
There was no encore performance.
The Pirates tagged Zambrano for four early runs and cruised to an 8-2 victory behind Ryan Doumit's 4-for-4, four-RBI performance. The doubleheader sweep served as an emphatic exclamation point after the Rockies officially eliminated the Cubs from playoff contention the night before.
"It was a very disappointing game," Zambrano said. "We weren't able to come back. The season is over for us, and we're going to work hard in the offseason and come back ready for next year."
Zambrano (9-7) finished the season with less than 13 wins for the first time since 2002, which also was the last time he made fewer than 30 starts.
"Look, he's had some injury problems," manager Lou Piniella said. "Everybody expects more out of him, including himself. So let's chalk this up as a season where he wasn't at his best. He's talked about working hard and having a big season next year, so let's just look ahead, not behind."
After he went on the disabled list for the second time this season, Zambrano admitted he'd "been lazy," about keeping in shape. After Wednesday's game, he outlined his offseason plan, which will include starting his conditioning program in November, earlier than he has in the past. He will then start a throwing program in January.
"The goal for next year is come healthy and come ready for Spring Training," Zambrano said.
On Wednesday, Zambrano's trouble began when Andrew McCutchen led off the game with a deep fly ball to left-center field, which fell for a triple when Micah Hoffpauir and Sam Fuld collided on the warning track. Two outs later, Lastings Milledge ripped a single to center to give the Pirates the lead.
The Cubs evened the score in the second on Mike Fontenot's RBI single off Jeff Karstens (4-5), but Zambrano walked two batters in the third and saw both come around to score on Ryan Doumit's double. Brandon Moss added a broken-bat RBI single to make it 4-1.
Chicago pulled to within two in the third when Zambrano doubled and scored on Ryan Theriot's squeeze bunt. In the sixth, they put runners on first and second, but pinch-hitter Jake Fox lined into an inning-ending double play.
Zambrano left after six innings, having surrendered four runs on six hits and four walks.
"We were expecting a better ballgame after the San Francisco start," Piniella said. "Didn't get it."
It didn't help that after Zambrano's double in the third, Chicago didn't record another hit until Derrek Lee's single with one out in the ninth. The Cubs had a total of 11 hits in the doubleheader, after Charlie Morton pitched his first career shutout -- a 4-0 win -- against them in Game 1.
"Two runs in two games, unless you pitch two shutouts, you're not going to win too many baseball games," Piniella said.
The Pirates added four insurance runs off the Cubs' bullpen, punctuated by Doumit's home run off Carlos Marmol in the eighth. It was the second home run Marmol has allowed this season in 73 innings.
"It's been a trying year, not only for me personally, but for the team as well," Doumit said. "Just to get a piece of mind to finish strong. Unfortunately, it's taken me all the way to September to kind of find my swing a little bit. It'll carry over."
Andrew Simon is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.