Cook, coming off a two-hit masterpiece against the Mariners, moves ahead of Franklin Morales, who has also pitched exceptionally well since moving into the rotation.
The reason the Red Sox are holding back Morales from a start until this weekend is so he can be available out of the bullpen Tuesday and Wednesday against Oakland. Lefty reliever Andrew Miller has been used a lot of late.
With the Yankees coming to Fenway Park this weekend, the Red Sox will open with Josh Beckett on Friday night. Morales and Felix Doubront will split the doubleheader on Saturday. Jon Lester gets the Sunday night finale, the last game before the All-Star break.