IMO, it's going to go to an internal candidate: Workman, Webster, RDLR, Ranaudo, Wright, Barnes, and Owens. I don't see what the point is in carrying these seven guys on the 40 man roster if they are not going to be used on the 25-man roster. Even if they trade one or two and bring one or two on to up to Boston, they still have plenty of organizational depth. So, I don't see the Sox investing additional resources for another more "established" arm with some type of risk (e.g., injury, recent ineffectiveness) to take that roster spot and potentially add to the existing redundancy. So, IMO, it's a matter of determining which arm is best suited for the job.
According to Sox Prospects, each of the potential internal candidates have options remaining, so that is not be a limiting factor. Therefore, it likely comes down to development and potential, and to a lesser extent ST performance. As I can only speculate in ST performance, I won't review it.
Development
Barnes and Owens likely benefit the most from continuing their development at AAA, as both really need to work on honing the command/control and improving their 3rd pitch. Webster and RDLR are further along in their development than Owens and Barnes and may benefit some from additional time in the minors to improve their command/control. Workman and Wright more or less are what they are at this point and probably either best off on the 25 man roster or in AAA as the first man up.
Potential
While Owens and Barnes may have the highest ceilings of this group over the long run, it's doubtful that they will be the most advanced pitchers in the group come April 1 and for them to be able to begin to reach their potential they need further development. Workman and Wright have the lowest ceilings of the group. Wright may turn into a successful knuckleballer, but that likely means his ceiling is a reliable #4 pitcher. Workman, particularly if he continues to struggle to maintain his velocity deep into games, likely has the ceiling of a reliable #5 starter. RDLR and Webster have the potential to be solid 2/3 starters if they can harness their stuff and become consistent.
Analysis
To continue their development, Barnes and Owens head back to Pawtucket where they anchor the AAA rotation and work on their control/command and 3rd pitch. Workman ends up in the bullpen. He was previously successful in a bullpen role and his stuff plays up there; I could see him being a steady 7th inning guy. I see Wright best fit for a role as a swing man/spot starter (knuckler w/rubber arm) or as the 1st man up in the case of injury to a starter (allows Barnes, Owens to not be rushed up). So, accordingly, and not surprisingly, the decision ultimately comes down to RDLR and Webster.
Conclusion
IMO, based on last year, it's RDLR's job to lose. However, if he struggles and Webster demonstrates that his last 4 starts last year are where he is trending towards then he may take that spot. If RDLR takes the 5th rotation spot, then Wright becomes the 7th man in the pen and serves as your long man and spot starter. If Webster takes the 5th spot in the rotation the RDLR goes to the pen where he works towards becoming a setup man/closer and Wright goes to AAA and is the first man up on the Pawtucket to Boston express.
Edit: I forgot to review Ranaudo in this post. IMO, he's best off on another team, specifically one with a big ball park. His fly ball tendencies and inability to throw down hill and work the bottom of the zone lead me to believe he's never going to be all that successful - as a reliever or a starter - in the AL East considering the ball flies out of the park in Baltimore, New York, Boston and Toronto.
