It's certainly not because of Clayton Mortensen's performance, but as the lone reliever with an option, he's the unlucky man to hop back on I-95 and south to Pawtucket. His demotion opens up room for Daniel Nava on the 25-man roster, but don't fret: Nava made it to the 40-man because the Red Sox designated Justin Thomas for assignment.
Nava was part of the all hands on deck approach of the 2010 season, when anyone who suited up for outfield duty was eventually injured. He hit .242/.351/.360 with a 91 OPS+, showing patience but a lack of pop, save that grand slam he hit in his first major-league plate appearance.
He's currently hitting .316/.421/.505 for Pawtucket in his age-29 season, and has been on an absolute tear the last 10 games (.447/.475/.737). While he hasn't hit lefties much, he does have an on-base over .400 against them this year, and has shown something of a reverse split against right-handers. That's not new, by the way, as he showed the same ability to hit pitchers of the same handedness back in 2010, both in the minors and in Boston.
Nava's main weak point involves his glove. Baseball Prospectus 2011* says that, on defense, Nava, "is lost: if his first instinct were to fall down and then run after the ball, it would look about the same as his current outfield strategy."
*To be fair, the author in question was I, and prior to that dose of truth, his intriguing story was told in detail. That glove, though. That glove.
The Red Sox now have Ryan Sweeney, Cody Ross, Darnell McDonald, Marlon Byrd, and Daniel Nava on the roster, giving them five outfielders for the first time in awhile. One wonders if Nava's ascension means McDonald's and Byrd's jobs are even more up in the air once Carl Crawford and Jacoby Ellsbury return. (Or, as Peter Abraham points out, it's possible one of the four outfielders is hurting right now, but not hurt enough to hit the disabled list.)
As for Mortensen, he can return to the roster in 10 days. If the Red Sox find themselves in need of a reliever capable of a few innings at a time, Junichi Tazawa's 10-day stint in Pawtucket will be at its end before the weekend is over, and Mark Melancon is certainly as ready to come back as Pawtucket can make him.