Manager Clint Hurdle announced the decision Tuesday at Fenway Park, where the Rockies worked out in preparation for Game 1, to be played on Wednesday night. Gaining Cook's services means rookie left-hander Franklin Morales will move to the bullpen and right-hander Taylor Buchholz will not be on the World Series roster. Buchholz was on the roster but not used the previous two playoff series.
Hurdle earlier announced his first two starters -- left-hander Jeff Francis in Wednesday night's Game 1 against Red Sox ace Josh Beckett and rookie right-hander Ubaldo Jimenez in Thursday's Game 2 against Curt Schilling. Hurdle also announced Tuesday that right-hander Josh Fogg will start Game 3 on Saturday at Coors Field. The Sox have not solidified their starters for the two games in Denver.
Cook, the Rockies' Opening Day starter, thought he was ready for the National League Championship Series after pitching in instructional ball in Tucson, Ariz., earlier this month, but the Rockies didn't feel he was sharp enough for a game.
"I'd been hurt and I wanted to be part of it, but the fact was they made a decision that they thought was best for the team," Cook said.
Hurdle said informing Cook he would not participate in the NLCS was his toughest conversation since taking over as the Rockies' manager during the 2002 season.
"It ended much better," Hurdle said of Tuesday's talk with Cook. "The opportunity to tell him, 'You're going to get the ball in Game 4,' was very special."
A simulated game and an intrasquad game after Cook was left off the NLCS roster helped the Rockies decide that Cook was ready. Cook went 8-7 with a 4.12 ERA in 25 regular-season starts, including a 3-1 mark with a 2.41 ERA in six starts after the All-Star break.
"It'll be like coming into the season at the beginning of the year where you have a couple of Spring Training games under your belt," Cook said. "My Spring Training games just happened to be rehab starts in the instructional league and a simulated game. I've got to be able to put those things aside and just go out and perform." Morales, who along with Jimenez was called up late in the year after injuries to Cook, Jason Hirsh and Rodrigo Lopez, showed an electric fastball while going 3-2 with a 3.43 ERA in eight regular-season starts. Morales started two postseason games, giving up three runs in three innings in Game 2 of the NL Division Series against the Phillies and one run in four innings against the Diamondbacks in Game 4 of the NLCS, before being removed both times when Hurdle felt he needed offense.
Cook, a second-round Rockies pick in 1997, has been an important figure in the Rockies' building program.
Cook appeared ready to become a force in 2005, but blood clots in both lungs flared and threatened his life during an Aug. 7, 2004 start. He had two surgeries, one to remove the top rib on the right side, and returned in late 2005 to go 7-2 with a 3.67 ERA.
Cook labored with low run support and a midseason slump last season, and finished 9-15 with a 4.23 ERA. He earned the Opening Day start this year, but struggled out of the gate before regaining his form at midseason.
After going on the disabled list in August, Cook was on schedule to return late in the regular season. But he reinjured the oblique during the first inning of a rehab start for Triple-A Colorado Springs at Tacoma on Sept. 1.
"At that point, it got a little low because I thought that might be the end of the season," he said. "But I kept working, hoping we'd make the playoffs."
The Rockies did, and they marched on to the World Series on the strength of a 7-0 record in postseason play thus far. And now, after gaining center fielder Willy Taveras for the NLCS round, the Rockies have added one of their front-line starters.
"I think we were able to handle it professionally and strategically," Hurdle said. "We all know the sentiments that go along with Aaron Cook and the value that he's brought to the organization, and time spent and all those things that have added value.
"But at the end of the day, No. 1, his health is no longer an issue for me. His pitches [during recent activity], he's shown what he's shown in the past."
Cook said facing the Rockies' regular lineup in an intrasquad game on Saturday put him where he needed to be.
Game 4 will mark Cook's second start of the season against the Red Sox. Cook pitched well on June 12 at Fenway Park, going 7 1/3 innings and giving up two runs and four hits, but the Rockies fell to the Sox and Tim Wakefield, 2-1.
Thomas Harding is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.