With lefty Andrew Miller likely out for the season following a fluke foot injury suffered last Saturday, the Red Sox acquired longtime White Sox bullpen stalwart Matt Thornton on Friday night to serve as a second lefty in the Boston pen along with Craig Breslow. Thornton had been with the White Sox since 2006, longer than any member of the 2013 squad other than Paul Konerko, and was once the top set-up man in baseball, by my accounting. From 2008 to 2010, the fireballing Thornton posted a 2.70 ERA (168 ERA+), 1.08 WHIP, 11.0 K/9, and 4.15 strikeout-to-walk ratio and made the All-Star team in 2010, but he has declined considerably since then.
This year, the 36-year-old Thornton's strikeout rate is down for the third straight year, dropping below league average to 6.8 K/9, while the four home runs he has allowed this year already match his top total since 2009. His 3.86 ERA (115 ERA+) is his worst mark since 2007 even though he has had more luck on balls in play this year than in any season since 2008. That all corresponds to a drop in velocity early this season, though Thornton has been picking up a pace more of late. Also of concern are Thornton's splits. Formerly a lefty that you could throw against any batter, he has been hit hard by righties this season to the tune of a .320/.414/.420 line. That could be a small-sample fluke, a theory BABIP supports, but his new skipper, John Farrell, may still want to limit the number of righties Thornton faces given his overall decline.