The interesting thing is that, relative to other pitchers, he has NOT overused his arm by any means in the first half. He's averaging 6.9 innings and 105.2 pitches per game, by no means excessive numbers. He's thrown a total of 1578 pitches, there's at least 70 pitches who've thrown more than that.
Having followed the Red Sox newsgroup on and off all season, many fans complained when Jimy Williams took him out after 6 or 7 innings.