"He's got inflammation in his right shoulder," a team spokesman said Friday, calling Cook "day-to-day."
The soreness showed up Thursday, and the Rockies decided to scratch Cook as a precautionary measure.
Jason Hirsh was already scratched from his appearance in Wednesday's game -- also with a sore right shoulder -- and has subsequently been bumped from his upcoming scheduled start Sunday.
Hirsh made 19 starts for the Rockies in '07 until a line drive in his final start fractured his right fibula and ended his season. Hirsh has been considered a favorite for one of the two remaining rotation spots, following Jeff Francis, Cook and Ubaldo Jimenez.
"Five or six years ago, we lost two starters in the last week of Spring Training," manager Clint Hurdle said Thursday, before news of Cook's scratch came out Friday. "We've tried to be smart with the additions that we make. We try to add value. We also try to create depth and versatility and find a way to sprinkle in some competition along with it.
"Then as the lay of the land progresses, sometimes it still gives you options when people are challenged, whether they miss a start in Spring Training, or however it might affect you when you go into the season. I think we're definitely in a position of depth."
Kip Wells, scheduled to follow Cook on Saturday, will make the start against the Royals.
Owen Perkins is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.