With Matt Thornton signing a new deal with the White Sox, it's likely he'll be the team's closer for the next few years. Thornton is pure fastball-slider reliever. He threw a fastball 87.4 percent of the time last season, using his slider mostly when ahead in the count.
Thornton gets a lot of velocity on his fastball, averaging 96.1 MPH last season, tops among all lefties. But the pitch also moves quite a bit. In 2010, his fastball averaged 5.7 feet per second of horizontal movement as it crossed the plate, good for 2nd among all lefties.
Thornton's only area of concern may be right handed batters. While righties only produced a .271 wOBA against Thornton last year, they had a 72.2 percent contact rate against his fastball, about 10% higher than lefties. Now that the league has seen his premier fastball, Thornton will have to start making adjustments in order to keep batters off their game. Look for him to start mixing in the slider a bit more in the closer's role this season.