But rookie right-hander Ubaldo Jimenez didn't see that as a reason to worry.
"That's the thing that makes you do better," he said. "You know they need you."
Jimenez, called up last month, delivered nine strikeouts and six scoreless innings, and another recent callup, Ian Stewart, drove in the winning runs in a 3-0 victory over the Padres at PETCO Park in front of 36,864.
Because the rookies showed that they could keep up, the Rockies gained ground.
They're still five games behind the National League West-leading Diamondbacks. But Colorado closed to two games behind the Wild Card-leading Padres going into Thursday night's deciding game of three between the clubs.
On Thursday, recent pickup Elmer Dessens replaces Cook, who goes on the 15-day disabled list to join Rodrigo Lopez (torn elbow ligament) and Jason Hirsh (cracked fibula). At 23, Jimenez (2-2) is old compared to 21-year-old lefty Franklin Morales, who could make his Major League debut Saturday on the road against the Dodgers.
But manager Clint Hurdle will worry about that later, and for now celebrate Jimenez overcoming two starts in which he yielded a combined 11 earned runs and 15 hits in 6 2/3 innings.
"I've got to believe I'm one of the people in here that hasn't done a lot of worrying," Hurdle said. "This will be a challenging week with the pitching situation. But, hey, we've got guys that are gonna get opportunities, and they're gonna go out there and get things done."
Jimenez, displaying the best fastball he's shown at the Major League level, didn't let a runner past second base. He held the Padres to one hit -- Geoff Blum's first-inning single -- two walks and a hit batter as he battled Padres starter Chris Young, who gave up three hits and struck out six against two walks in six scoreless innings.
Jimenez's nine strikeouts matched then-rookie Jason Jennings' performance against the Marlins on May 15, 2002. The club rookie strikeout record is 13, by Shawn Chacon on June 7, 2001, against the Astros.
"He throws hard," the Padres' Termmel Sledge said. "We hit some balls hard that didn't fall. It's just one of those games you can't explain. We were seeing him good, but, hey, tip our hat to him."
Wednesday had its rough patch. With two down in the fifth, Jimenez inexplicably walked Young on four pitches and wild pitched him to second. Brian Giles stung a ball, but it was right into the glove of shortstop Troy Tulowitzki.
"I talked to myself, like, 'Come on, relax,'" Jimenez said. "I walked the pitcher, but so what?"
Stewart ended the scoreless duel with a two-out, two-run, two-strike pinch-hit single off Cla Meredith (4-6) in the seventh. Since being called up on Saturday, both of Stewart's hits have been pinch-hits -- a double off the Cubs' Kerry Wood as part of a game-turning rally Sunday and the game-winner Wednesday night.
"Now it's just baseball again," said Stewart, the Rockies' top Draft pick in 2003. "I try to tell myself that before each at-bat to help calm myself. Just go out there and play."
Jamey Carroll added a pinch-hit single and scored on a Kazuo Matsui single, both off Doug Brocail, in the ninth.
Rockies reliever LaTroy Hawkins gave up a two-out hit -- to end a streak of 18 batters retired -- in a scoreless seventh.
Jorge Julio pitched a perfect eighth to run his scoreless-inning streak to 9 2/3.
Manny Corpas overcame a hit and a walk with one out in the ninth by forcing a Sledge line drive to center and a Kevin Kouzmanoff infield bouncer to improve to 10-for-10 on save chances.