Chacin winced and put his left hand to his back after a 91 mph, four-seam fastball to Cody Ross with one out in the seventh. The next pitch was a 90 mph fastball. He gripped his side and walked around the mound in pain before being removed from the game.
"I don't know, it was kind of right in the middle -- could be my oblique, could be my back," Chacin said. "I just felt something pull."
Chacin's effort on Friday keyed the Rockies' seventh straight victory. Chacin, 25, in his first year as the No. 1 starter, has been a big part of that. Chacin (3-0) struck out five and held the D-backs to three hits and a walk, plus drove in a run with a fifth-inning sacrifice fly. By night's end, his ERA was down to 1.46.
Chacin's biggest inning was the fifth. Protecting a 2-0 lead, Chacin retired the first two batters before A.J. Pollock and Didi Gregorious singled, and Cliff Pennington drew a walk. But Chacin forced D-backs pitcher Ian Kennedy into a grounder to short.
"I'm a little bit worried," he said. "I don't want to miss some starts. I just want to pitch, especially how we're playing right now. I just want to keep helping my team."
It's the first challenge on a surprisingly smooth early season for the Rockies, who at 12-4 have matched the 2011 squad their best start through 16 games in the club's 21 seasons.
"We hope it's nothing serious," Rockies third baseman Chris Nelson said. "We don't want to make any speculation, but when you see a pitcher walk off the mound it's a rough one for the whole team."
Chacin said he felt better Friday night after receiving ultrasound, massage and ice treatments, but Saturday's examination will tell him more.
The Rockies don't have the luxury of a day off, so skipping Chacin to give him extra time is not an option. A candidate at Triple-A Colorado Springs is left-hander Drew Pomeranz, who is 2-1 with a 3.31 ERA in three starts, with 21 strikeouts and five walks. Pomeranz last started on Thursday.
It's a bad time for a mysterious injury.
Last year, Chacin struggled to 0-3 with a 7.30 ERA through five starts. His velocity and location were poor, but he couldn't pinpoint what was wrong. It wasn't until after the Rockies attempted to send him down to Triple-A Colorado Springs that Chacin had the club investigate his health. It took some time after that to find a nerve injury in the right side of his chest. He didn't return until August, and went 3-2 with a 2.84 ERA in his final eight starts.
This spring, he had a start late in the Cactus League season scratched because of back spasms. Chacin suffered those while doing squats. He has since cut squats out of his fitness routine.
It's also possible that the events leading to Chacin's last start, a mediocre but successful five innings in San Diego, was a harbinger of pain. The Rockies arrived two days before his first start, which meant three nights of rest that turned out to be not-so-comfortable.
"The beds in the hotel weren't that good," Chacin said. "I kind of felt tight. It wasn't until the third inning I felt a little bit loose. It was hard when I started an inning, starting to warm up I felt really tight. But after a couple of pitches I got loose.
"Today it was fine, I didn't feel anything until the last two pitches."