"Aaron Cook lost his rotation spot, but because of injury more than ineffectiveness. Actually, to hear him talk Thursday, the ineffectiveness was related to his sprained right big toe.

It had been quietly bothering him for three weeks, the right-hander keeping it under wraps because he didn't want to make any excuses for his poor outings. But after Tuesday's loss to the Giants, there was no denying the obvious.

"I wasn't helping the team," Cook said.

Promising rookie Jhoulys Chacin was expected to fill Cook's spot but started Thursday for the Sky Sox, throwing more than 60 pitches that could knock him out of consideration. That leaves Esmil Rogers as the likely starter Sunday.

Cook will play catch this weekend, possibly throwing a side session before joining Double-A Tulsa Monday to begin a rehab assignment.

Because a starter isn't needed until Sunday, the Rockies promoted infielder Chris Nelson for a three- day cameo. Nelson, who was hitting .329 with 11 home runs for the Triple-A Sky Sox, received the call over Jonathan Herrera, who was unable to return because of a rules issue involving his optioning earlier in the week.

This is the second consecutive season that Cook's toe has landed him on the DL. He altered his mechanics a year ago when it started hurting, causing shoulder pain. When he began throwing across his body against the Giants, he realized it was time to shut it down.

"We wanted to be proactive about it," manager Jim Tracy said of Cook, who owned a 13.97 ERA over his past three starts. "It's not to fair to the club or the player to send him back out there like that.""