Third baseman Bill Mueller was scratched from last night's starting lineup because of a stiff back, prompting the Red Sox to summon third baseman Kevin Youkilis from Pawtucket. To make room for Youkilis, the Red Sox designated newly acquired outfielder Jose Cruz Jr. for assignment. Mueller, sitting stiffly in the chair in front his locker, a big towel-covered ice pack strapped to his lower back, said he was hoping to be able to play tonight.