I'd say if the Red Sox are going to upgrade...look at the positions they need to upgrade.
SS, CF, and C are lacking in the offensive category. Both Ellsbury and Lowrie have upside but it's up to the Red Sox front office to evaluate there true ceiling.
If not:
SS (Hanley Ramirez, Jose Reyes, JJ Hardy, Troy Tulowitzki, Yunel Escobar)
CF (Grady Sizemore, Josh Hamilton, Carlos Beltran, Ichiro Suzuki)
C (Brian McCann, Russell Martin, Joe Mauer, Victor Martinez, Salt, etc etc....we need something)
With a loaded farm system and young players on the major league roster (plus a couple veterans that could be moved) the Red Sox have an opportunity to make something happen.