While "elbow strain" is the official reason, it seems just as likely that Lackey is out for mental health reasons. The turbulent state of his personal life has been well-publicized of late, with many speculating that it had been the cause of his poor performance on the mound.
Scott Atchison, who was spotted in Fenway earlier but said to just be picking something up, was apparently there for his Red Sox uniform, as he will be the man replacing Lackey on the roster. It's often the case in situations like these that the man called up will be sent back down on the following day in exchange for a spot starter, though Atchison was tasked with that duty during a game last year against the Phillies. Alternatively, Atchison could stay with the team with Tim Wakefield getting the start for the Sox.