The Red Sox have acquired outfielder Alenjadro De Aza from the Orioles, according to Ken Rosenthal. It's not yet known what the Sox gave up, but given that De Aza was on waivers after being designated for assignment by Baltimore, it's hard to imagine it's much.
[UPDATE]: The Sox seem to have given up cash, and nothing more.
A few years ago, De Aza was a solid everyday player for the White Sox. But he did not impress in Chicago last year, and while he enjoyed a brief resurgence after moving to Baltimore mid-season, that renaissance has not lasted into 2015. De Aza is hitting just .214/.277/.359 in 112 plate appearances for the Orioles.
In other words, he should fit right in.
In all likelihood, De Aza is just the latest 25th man for a roster which has already had its fair share. He will probably replace Carlos Peguero for the immediate future, and then make way for, say, Shane Victorino when he's ready to return to action. Playing time in the interim will be limited mostly to emergency fill-ins and the occasional pinch-running appearance.