We don't need Koji to be as good as he was in 2013. Koji can still be a very good and effective closer without reaching his 2013 level. Mujica was very impressive after the All Star break last year. Taz should be solid and consistent. Between Breslow, Ogando, Layne, Varvaro, Barnes, Workman, Escobar, Hembree, Spruill, and Ross, we should be able to round out a decent pen. There may even be a diamond in the rough amongst those guys. If not, the Sox certainly have the means to trade for a reliever before the deadline.
As far as starting at a bigger workload for the pen, the Sox starting staff in 2013 averaged 6.1 innings per start. For some reference, league average that year was 5.9 innings per start. Our projected starting 5 averaged the following # of innings per start last year:
Buchholz - 6.1 (6.8 in 2013, so he could conceivably do better than 6.1)
Porcello - 6.5
Miley - 6.1
Kelly - 5.7
Masterson - 5.0 (6.5 in 2013, so he should conceivably do much better than 5.0 if healthy)
Provided the rotation stays fairly healthy, I'm not seeing the bigger workload.