Zambrano left the game in the second with lower back stiffness and will get an MRI Tuesday. The 25-year-old has had problems with his lower back in the past but this is the first such instance this season.
"I couldn't throw today," said Zambrano. "I couldn't bend today, and as a pitcher, you have to bend as much as possible to get the strike zone and to get your command. If you throw straight, you let your pitches go high and that's what happened today."
It was the shortest outing of his career. After a meeting with the trainer on the mound, Zambrano (14-6) left the game trailing 5-0. He gave up four earned runs, four hits and four walks and lost for the first time in five starts. Zambrano hasn't missed a start, has thrown 193 innings this season and has been the only constant in the rotation.
Zambrano said he requested the MRI but he didn't feel anything more serious that muscle tightness and spasms. If he were to go on the DL, he would join teammates Mark Prior and Kerry Wood, who have missed most of the season with injuries.
"There are certain things that we look for," Cubs trainer Mark O'Neal said. "It seems like it's all muscular. He's not having some of the other symptoms you would be worried about with significant disc injuries."
The Pirates have won four straight against the Cubs in a battle of the NL's two worst teams and are just a half-game back of the Cubs for fifth place in the NL Central.
After Pittsburgh reliever Salomon Torres got the last out in the ninth, he jumped off the mound in celebration. Other Pirates
celebrated the win.
"I don't want to finish in last place, we don't want to finish in last place," Cubs first baseman Derrek Lee said. "But those are bad goals to have. For them to be getting excited about getting out of last place, I don't know how much difference it really makes. But I don't want to finish in last place."
Paul Maholm (7-10) gave up four runs, three hits and three walks in five innings, retiring 10 of the last 11 hitters he faced. He has won three straight decisions.
"We want to get out of the cellar," Maholm said.
John Grabow, Damaso Marte and Matt Capps kept the Cubs scoreless in the next three innings and Torres got his second save in the last three days. He previously hadn't picked up a save since Oct. 2, 2005. The Pirates' bullpen has allowed just one run over its last four games, a span of 12 innings.
"That group of guys was terrific, to keep them from scoring for the remainder of the game," Pirates manager Jim Tracy said.
Zambrano struggled with his control from the beginning, walking the first two batters before missing a tag on Freddy Sanchez after his bunt. Jason Bay followed with an RBI bloop single and Nady cleared the bases with a double to right-center.
"Everyone knew he wans't throwing as hard as he normally does," Bay said. "We thought he wasn't as loose as he usually was and he was kind of laboring through that first inning."
"I feel like my back locked," Zambrano said.
Ryan Theriot had a three-run double in the second as the Cubs cut the lead to one. Maholm loaded the bases when he hit rookie Scott Moore in his first major-league at-bat. Juan Pierre had an RBI single to start the four-run inning.
No one scored after that as the bullpens took over.
The Cubs activated right-hander Carlos Marmol from the 15-day DL and recalled Moore from Triple-A. Marmol, who had been starting
before going down with shoulder fatigue, will be used out of the bullpen. ... Cubs catcher Michael Barrett was still in the hospital Monday morning for a follow-up procedure after having surgery Saturday night to stop bleeding in his scrotum. He is on the 15-day DL after getting hit in the groin area by a foul tip on Saturday. ... Pirates shortstop Jack Wilson missed his 10th straight game with symptoms from an inner ear infection.