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DENVER -- Chicago Cubs reliever Sean Marshall was putting on his uniform in the clubhouse when he got word of his unexpected start Friday night against the Colorado Rockies in place of Carlos Zambrano.

"I rushed to get dressed and get loose," Marshall said. "That's the first time I've done that. It was tough."

The Rockies took full advantage of the late scratch of Zambrano because of a lower back injury, jumping on Marshall for three runs in two innings en route to a 6-2 victory in the opener of a four-game series.

"It was a big deal," said the Rockies' Todd Helton, who singled twice to drive in two runs and extend his hitting streak to 12 games. "We ate up their bullpen a little bit. It should help us the rest of the series."

About 15 minutes before the start of the game, Zambrano was scratched because of a recurrence of the lower back injury.

Zambrano, who was 7-4 with a 3.35 ERA entering the game, worked a season-low three innings in his last start against Florida on Aug. 1, before leaving the game with lower back stiffness. He had worked out with no complaints since then and Cubs manager Lou Piniella had said before the game that there were no concerns about Zambrano's back.

"When he went out there to warm up, he says after he threw two or three pitches, his back tightened up and he started getting spasms," Piniella said. "This is something that happened right before the ball game. If he had been complaining about anything, obviously, we wouldn't have scheduled him to pitch."

Zambrano was sent back to the hotel and was reported to still be in discomfort. Piniella said he would be sent to Chicago on Saturday and undergo an MRI. It was still uncertain whether Zambrano would go on the disabled list for the second time this season.

"We'll go off the [MRI] results," Piniella said.

Said Marshall, "I hope 'Z' is all right and whatever injury he went down with is something he can get over quickly because we need him in our rotation. He's one of our mainstays and one of the guys that usually gets it done for us."

While the Cubs were scrambling over their pitching staff, the Rockies got a solid start from Ubaldo Jimenez (9-9), who allowed two runs -- one earned -- and four hits in 6 2/3 innings for his third consecutive victory.

"My first thought was `It's too bad for him,'" Jimenez said of not having to go against Zambrano. "I hope he wasn't hurt too badly. It's good for us. But I had the same mental approach. I still had to compete no matter who was pitching for them."

Chris Iannetta hit a two-run homer in the fourth off Jeff Samardzija, helping the Rockies to their sixth win in eight games and 15th straight against a team from the NL Central.

Marshall, who used to be a starter, gave up four hits in the three-run first, when Helton had the first of his two RBI singles. Troy Tulowitzki followed with a double and Brad Hawpe had a two-run single to put the Rockies up 3-1.

Ian Stewart singled to lead off the fourth and Iannetta drove Samardzija's 3-1 offering into the Rockies' bullpen in center field for his 13th homer.

The Cubs added an unearned run in the sixth when second baseman Omar Quintanilla failed to handle shortstop Tulowitzki's throw attempting to force Milton Bradley on Derrek Lee's fielder's choice. Bradley scored on Alfonso Soriano's two-out single.

Jimenez, who struck out six and walked five, was touched for his only earned run in the first on Lee's double-play grounder. The game marked the ninth consecutive start in which he has allowed no more than four runs and no less than two runs, going 3-3 in that span.

Game notes

Slumping Rockies 2B Clint Barmes, in a 2-for-46 slide, was given the night off. ... Rockies OF Carlos Gonzalez was moved up from his usual eighth spot to No. 2 in the order. ... The Cubs activated C Geovany Soto (left oblique strain) from the 15-day DL and optioned INF/OF Micah Hoffpauir to Triple-A Iowa. Soto struck out three times in his return. ... The Rockies last lost to an NL Central club on June 3, at Houston.