It would be reckless to go into 2014 relying on Koji to pitch another 75 straight shutout innings. His 2013 season was possibly one of the best seasons for a relief pitcher ever; the chances that he'll even come close to that again are pretty slim.
On the other hand, he Sox wouldn't want to sign a closer who expects to be a closer, since Koji has earned the right to start the season there. What they need is an excellent set-up man who is ready to slide into the closer role if/when Koji falters. I'm skeptical that Junichi is that guy, and I know neither Bailey nor Hanrahan are.
So where should they go? Bullpens are finicky, and there's a lot of matriculation throughout a season. A player can be stellar one year, and bring a 5.50 ERA the next. A guy can be lights out for two months, and then a strained forearm takes him out the rest of the year. Or the reverse; a guy can flounder along in the 6th inning for 10 innins with a 6.00 ERA, and then suddenly *click* and be lights out the rest of the season.
So obviously, the Sox want to stock up on low-cost middle relief and see who might emerge. But should there be any big name targets in trade or FA? Are you satisfied with just relying on the prospects? What's a GM to do?