Cook will undergo an MRI on Monday to pinpoint the nature and severity of the injury. Rockies manager Jim Tracy said he hopes Cook's injury is similar to the one left-hander Franklin Morales suffered early in the season, which could mean he'll be out 3 1/2 weeks. That would leave just half of the final month remaining if Cook needs an injury rehab assignment.
Cook (10-6) left Friday night's game against the Giants after a fourth-inning leadoff walk to Nate Schierholtz. He was charged with four runs on three hits and the loss in a 6-3 Rockies defeat.
Filling Cook's roster spot, the Rockies purchased the contract of right-handed reliever Matt Herges, who pitched for the club in 2007 and '08, from Triple-A Colorado Springs. Herges began the season with the Indians and went 2-1 with a 3.55 ERA in 21 appearances before being released on July 8. Herges signed a Minor League contract with the Rockies on July 17 and went 3-2 with a 1.96 ERA in 13 appearances with Colorado Springs.
Cook, 30, had suffered from a hyperextended right big toe in the weeks before Friday's injury. But as Cook said on Friday, Tracy reiterated Saturday that the shoulder injury is not thought to have been caused by any mechanical change brought on by the toe injury.
"It was a one-pitch-type thing," Tracy said. "That was encouraging from the standpoint that structurally, more times than not, there's lingering stuff. This has not been a lingering thing by any means."
Cook's injury is not believed to be as serious as the strained oblique muscle that cost him the end of the 2007 regular season. But the similarity is that the Rockies will have to play September games without one of their most-accomplished pitchers.
In addition, the Rockies placed right-handed reliever Manuel Corpas on the 60-day disabled list. Corpas underwent surgery to remove a bone chip from his throwing elbow on July 24.
Corpas revealed on Saturday that he underwent another surgery on Aug. 12 because of swelling in his right forearm. There were no additional chips in the arm, but an infection had set in, so Corpas is taking antibiotics through a pump every 10 hours. The follow-up surgery makes it doubtful that Corpas will make a regular-season comeback this year. Corpas said he hopes to pitch in the playoffs if the Rockies qualify, but he also said the club could shut him down and possibly not clear him to even pitch in winter ball.
The Rockies entered Saturday night's game -- the second of four with the Giants -- leading San Francisco by one game in the National League Wild Card race and needing a victory to remain 4 1/2 games behind the NL West-leading Dodgers.
Thomas Harding is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.