DENVER -- Two of the strangest chapters in this never-ending saga of a Cubs season occurred Friday night.

Carlos Zambrano walked off the field 15 minutes before his scheduled start against the Rockies with more back problems, and that overshadowed the last-gasp move of Milton Bradley into the No. 2 slot in the lineup.

"I'll tell you, this has been an experience," exasperated manager Lou Piniella said about the season of injuries and inconsistencies.

This latest experience included a predictable outcome, with emergency starter Sean Marshall losing 6-2 to the Rockies and dropping the Cubs one game behind the Cardinals in the National League Central.

And it might not get better, with Zambrano scheduled for an MRI Saturday after he returns to Chicago, one that will determine whether he will be forced to the disabled list for a second time this season.

He left his last start with back stiffness, Saturday in Florida, after three innings but hadn't complained of discomfort.

"He hasn't gotten any treatment [all week]," Piniella said.

Zambrano might have strained his back while swinging the bat against Florida, and he took batting practice Friday before the same problem arose, although Piniella wanted to wait for MRI results before making any judgment.

Zambrano returned to the team hotel and was not available after the game, but his absence left the clubhouse somber.

"Life has thrown us some curves," said Marshall, who was getting dressed when called into action.

Marshall pitched two innings and left with a 3-1 deficit, which was stretched by two more runs in the fourth inning with Jeff Samardzija pitching.

The Rockies' three first-inning runs, which came on four hits, ruined Bradley's debut as No. 2 hitter. He had spent most of his time in the middle of the batting order, where he produced only 26 RBIs, but he was also second on the team in walks to Kosuke Fukudome, who batted in front of him Friday.

Ryan Theriot has batted No. 2 most of the season but was dropped to No. 8.

"This has nothing to do with Theriot," Piniella said. "We're trying to get Bradley going, utilize his on-base percentage."

Fukudome and Bradley each finished with a single and two walks, although 3-4-5 hitters Derrick Lee, Aramis Ramirez and Geovany Soto didn't hold up their end. They were a combined 0-for-11.

"[Bradley] got on base three times, which is what we want, but it creates problems other places in the lineup," Piniella said. "We're trying to find a lineup that will produce runs consistently. That has been hard to do."

The Cubs need to score runs if they are going to stay in the division race. And now the wild card race. The Rockies are ahead of the Cubs in that one.

Plus, when the Cubs return to Wrigley Field on Tuesday after a 10-game trip, their welcome-home gift is the world champion Phillies.

dvandyck@tribune.com Up next Saturday at Rockies, 7:10 p.m., CSN