Something's got to give.
The issue does not lie in the outfield, currently. Though it was fully stocked recently enough, the departures of Darnell McDonald to New York and Ryan Kalish to Pawtucket have opened up plenty of space there. Currently the Sox only have one backup outfielder in Brent Lillibridge, while the likes of Mauro Gomez, Pedro Ciriaco, Will Middlebrooks, Nick Punto, Adrian Gonzalez, Mike Aviles, and the aforementioned "Mr. Versatile" in Lillibridge provide excess depth in the infield.
If it were as simple as adding Ellsbury to the 25-man roster and sending Mauro Gomez back to Pawtucket (and he likely would be the choice, as much as many here would like to see Lillibridge or Punto go), then this could be taken care of as easily as the Kalish move was. But it's not, because Ellsbury is not simply rejoining the 25-man roster, but also the 40-man, where there is no room to go around an no clear choice to be designated for assignment.
While there are plenty of players on the 40-man roster who could be dropped without any impact on the depth chart as it stands, there's nobody there so inconsequential that the Red Sox will want to give them away without receiving an appropriate return. Though the likes of Lars Anderson and Che-Hsuan Lin are certainly not the types who can headline trades, they can be decent throw-in pieces to push a deal over the edge or acquire long-shot talent like Rich Harden would have been last year or Jarrod Saltalamacchia was the year before.
Lin and Anderson, however, don't seem the most likely to go. Instead, it sounds like the Sox will turn to the same place they're adding depth for a trade chip: the outfield.
With Jacoby Ellsbury back, the outfield is the place where the Sox have the most valuable excess, as it were. By name: Ryan Sweeney.
When the year first began, the idea was to platoon Cody Ross and Ryan Sweeney in right field, with Jacoby Ellsbury in center and Carl Crawford in left. Since the season started, however, that's mostly changed to playing them alongside one another when healthy with Daniel Nava in left. Now that the injured players are returning, however, the platoon no longer seems quite so attractive thanks to Cody Ross' exceptional year-to-date featuring comparable numbers against right-handed pitchers.
If Carl Crawford's return wasn't quite so imminent, the Sox could justify holding onto Ryan Sweeney for a while longer, but as is Daniel Nava is set to become the fourth outfielder, and even with his recent slump he's massively outperforming Sweeney. Add in the presence of Scott Podsednik as Triple-A depth, and there's just not much place left for the Oakland import.
The real kicker, though, is that Sweeney could actually bring some return on his own. We're not talking a major trade by any stretch of the imagination, but as a young outfielder with good defense at all three positions and a bat that has shown some serious doubles power this season (if that is, in fact, a thing), Sweeney isn't going to be the sort of guy you pick up off waivers. Having looked at the outfields around the majors during the Darnell McDonald designation, it was clear that there were plenty of teams that could use someone like Sweeney.
There will be more shoes to drop as the month progresses and the likes of Carl Crawford also make their way back on the 40-man roster. For now, though, Sweeney seems the favorite to be the first to go.