ESPN has its All-Star projections up, and not surprisingly, Jon Lester isn't among them. I can't muster a great argument for him, but conversely they project Daisuke Matsuzaka as making the squad. International politics aside, I don't see that as being likely either.
............... IP WHIP K/BB ERA
Lester.... 117.2 1.31 1.85 3.21
Matsuzaka 75 1.39 1.33 3.12
Note that won-loss record isn't included as it isn't a highly reliable measure of effectiveness, with miscellaneous components such as run support and bullpen support critical elements.
Since April, Lester has pitched 78 innings, allowed 75 hits, and his strikeout to walk ratio has been 54-18 (3). During that time his ERA has been 2.65 and his WHIP ratio (walks plus hits per inning pitched) fallen to 1.19.
In other words, Lester has given the Sox more innings, better innings (by WHIP and K/BB), and is going deeper into games. Even with the smaller sample size, Matsuzaka has not outperformed Lester.