Cook could have -- and would have -- exercised his Tuesday opt-out clause if there was no spot for him on the Major League roster.
But with Beckett feeling some soreness in his right lat muscle after equaling a career high of 126 pitches Sunday vs. the White Sox, the Red Sox found a way to get Cook a start.
The expectation is that Beckett will miss just one start and return to the rotation on or around May 10 against the Indians.
"I don't think there's any concern about him making his next start or even if he could have with the extra day, made this start," said manager Bobby Valentine. "It would have been a situation we would have had to watch carefully, so there's no need."
In fact, with the Cook decision coming to a head, the timing was good to be conservative with Beckett.
"It was a factor," general manager Ben Cherington said. "We're not concerned about Josh. At different points of the year, he might pitch with it, but we just wanted to be cautious and get ahead of it. Once we decided to do that and started lining up all the factors, we felt like we didn't want to lose good pitching depth this early in the season."
The 33-year-old Cook, a veteran sinkerballer who had spent his whole career with the Colorado Rockies, was impressive in his Minor League stint at Triple-A Pawtucket, going 3-0 with a 1.89 ERA in four starts.
At this point, the Red Sox aren't sure how they will utilize Cook after his spot start. But with the 20-game, 20-day stretch which starts Friday, it's conceivable he could give different pitchers in the rotation an extra day of rest.
"I don't think there's any need to really project anything further than this Saturday. We'd like to see him in a Red Sox uniform in the big leagues and take it from there," said Valentine, who alluded to the upcoming grueling stretch. "I'm not opposed to having more than five starters during a long stretch like that depending on how the starters are pitching."
Cook signed a split Minor League deal with the Sox during the offseason that included two opt-out dates, Tuesday and June 1. Cook will receive a prorated salary of $1.5 million in the Majors, plus bonuses for the number of appearances he makes.