"Even for Aaron Cook, patience has an expiration date.

He is the franchise leader in wins. And no pitcher has spent more time in a Rockies uniform. But in a pennant race, the past and tenure don't necessarily matter. Challenged to give his team a chance to win Tuesday, Cook dissolved again before the Rockies' eyes in an ugly 10-0 loss to the Giants, their worst at home this season.

The performance jeopardized his rotation spot after Colorado dropped 6 1/2 games behind San Francisco in the wild-card race.

"Every team is giving me trouble this year," Cook conceded. "I have been pitching badly."

Manager Jim Tracy elected not to discuss Cook's status afterward, saying there would be opportunities later, possibly as soon as today. But the raw numbers and Jhoulys Chacin's presence as a potential replacement in Triple-A Colorado Springs suggest that a change is imminent.

Cook is 0-3 in his past three starts with a 13.98 ERA. Teammates wondered late Tuesday if he had already been told he's moving to a long relief role.

"If I had the answer (to the command issues), I would fix it," said Cook, whose control is fine in sessions between starts. "It's aggravating."

Twenty-eight pitches into Tuesday's outing, Cook was knee deep in trouble. He had retired only one batter. The Giants finished with four runs in their initial at-bat, going 5-for-6 with runners in scoring position, including a single by wonderboy catcher Buster Posey.

It continued an alarming trend for Cook. While his velocity is similar to that of his last few seasons — 88 to 91 mph — Cook's strong sinker remains elusive. When he misses, it's elevated. And he doesn't have the offspeed pitches to compensate, his 70 mph curveball to fan Posey in the third inning notwithstanding. Cook dropped to 4-8 with a 5.34 ERA. Chacin, before his demotion when Jorge De La Rosa returned to the rotation, was 5-8 with a 4.04 ERA. "