With four starters with ERAs over six (Lester, Beckett, Penny, and Matsuzaka), one has to scratch his head and wonder how the Sox have stayed close in the AL East. The Sox are 21-12 and only a game out.
But it's not difficult. Offensively, the Sox are second in the AL in scoring, about 5.7 runs per game, and the pitching is tenth in the AL in ERA. The Sox are sixth, however, in strikeouts to walks, and Red Sox data shows that K/BB ratio predicts future ERA better than ERA itself. Still, more telling is the Red Sox bullpen, that is third in ERA at 3.20 and leads the AL in saves with 11. With the AL overall bullpen ERA at 4.54, that's quite a differential.
This has translated into a 6-4 win-loss record in one-run games (good, not great) and their overall record exceeds their expected record using baseball's Pythagorean Theorem.
Manny Delcarmen, Ramon Ramirez, and Jonathan Papelbon have ERAs of 0.52, 0.52, and 1.20 respectively, allowing four earned runs in 49 2/3 innings. They have allowed only 33 hits and 22 walks with 42 strikeouts.
Certainly, we can't expect the bullpen to have the collective success they've enjoyed so far, but consistent offense and superior relief have allowed the Sox to survive mediocre starting pitching.