1) Matt Thornton, White Sox
Fireballing lefty Matt Thornton is the highest ranking non-closer in Baseball Prospectus' opposition-adjusted win-expectation-based relief pitcher statistic WXRL. If that doesn't move you, his traditional stats should: 2.44 ERA, 1.04 WHIP, 11.0 K/9, 4.2 K/BB. A former first-round draft pick and minor league starter, Thornton's wildness out of the Mariner pen led Seattle to trade him to the south side for Joe Borchard in March 2006, and he's been a Second City mainstay ever since. He had a breakout season at age 31 last year and has been even better this year, shaking the LOOGY role for proper setup work in early June. Since July 1, he's posted a 1.71 ERA, 0.86 WHIP and walked just four men in 21 innings for a staggering 6.5 K/BB.
2) Phil Hughes, Yankees
The Yankees' top pitching prospect prior to the arrival of Joba Chamberlain, Phil Hughes made his major league debut in early 2007, but spent most of that season and the next fighting injuries (though none to his pitching arm) and resultant poor performance. Having pitched well in spring training this year, Hughes got the call when Chien-Ming Wang imploded in the seasons' early weeks and pitched well enough in the rotation to stay on the major league roster upon Wang's (ultimately aborted) return. Pitching regularly in relief for the first time, Hughes quickly rose to the eighth inning and twirled off a streak of 25 scoreless innings. Dating to the beginning of that streak, he has posted a 0.91 ERA, 0.91 WHIP, 11.52 K/9, and 4.75 K/BB in just shy of 30 innings. In the process he's proven that Joba Chamberlain's success in the same role was not so preciously unique that it was worth sacrificing his potential as a starter, though you can expect the same sturm und drang over Hughes' return to the rotation next spring.
3) Matt Guerrier, Twins
The Warrior has been a key part of the Twins' bullpen since 2005, but this year he has simply stopped walking people, issuing just nine free passes all season. Guerrier's 1.4 BB/9 has led to a 4.33 K/BB and 0.85 WHIP that helps support his 2.30 ERA. Since May 1, he's been even better, posting a 1.66 ERA and earning 21 holds. And since June 1, that ERA has been just 1.34, making Guerrier and Joe Nathan a devastating late-game combination.
4) Mike Gonzalez, Braves
Gonzalez opened the season as the Braves' closer, but Rafael Soriano lurked, ready to pounce on his smallest mistake. A pair of blown saves in mid-May was all Soriano needed to overtake his 'pen-mate, but Gonzalez has excelled as Soriano's setup man since. Since June 1, the former Pirates closer has posted a 2.12 ERA with 10.85 strikeouts per nine innings and just two home runs allowed. A free agent after the season, he's sure to land a high-paying closer job for 2010.
5) Hideki Okajima, Red Sox
History shows that Japanese pitchers tend to offer diminishing returns as the league becomes familiar with their stuff and their often-funky deliveries. Indeed, Hideki Okajima's ERA has shot up over his three seasons in Boston's 'pen from 2.22 in 2007 to 2.82 this year. That is to say that the lefty changeup artist is one of the most reliable eighth-inning setup men in the game. His rate stats over his first three seasons have shown very little variation and he ranks third among non-closers in WXRL this year. Since May 1, he's posted a 2.18 ERA with 19 holds.
6) Ryan Madson, Phillies
A minor league starter, Madson emerged from several years of indecision regarding his role to become a dominant setup man for Brad Lidge last year as the Phillies closed in on their second-ever World Series title. This year, he's back at it, throwing peas past National League hitters in service of a Phillies team that stands a very good chance of returning to the Series.
7) Rafael Betancourt, Rockies
A former Red Sox minor leaguer, Betancourt was a scrap-heap find by the Indians in 2003. Over the next five seasons, he posted a 2.80 ERA, 9.1 K/9, 1.06 WHIP and 4.6 K/BB. A poor 2008 and the Indians' struggles this season took him off the radar, but in 11 appearances since being traded to the wild card-leading Rockies, he has yet to allow a run (and he's allowed just one in 15 appearances since returning from a groin injury in early July, posting a 0.66 ERA and 0.73 WHIP over that stretch).
8) George Sherrill, Dodgers
The Dodgers eighth-inning situation is a curious one. Sophomore Ramon Troncoso had it locked down for most of the year, boasting a 1.67 ERA on July 20 and ranking above Okajima in WXRL despite an unimpressive 1.71 K/BB. Since then, however, he's allowed 17 hits in just nine innings, posting an even 10.00 ERA and losing both his role and Joe Torre's trust (his last six appearances have all come in Dodger losses). Enter deadline acquisition George Sherrill, the former Mariners LOOGY and Orioles closer. Sherrill's peripherals haven't been especially impressive since he has donned Dodger blue, but he has yet to allow a run in 9 2/3 innings since the trade, bringing his season ERA down to 1.94.
9) Brad Ziegler, A's
Speaking of former closers, Ziegler opened the season as Oakland's closer, but lost the job not through any failure of his own (he only blew one save), but because of Andrew Bailey's undeniable dominance. What goes around comes around, as Ziegler did the same to Huston Street last year. Ziegler has since settled into the eighth inning, using his sidearming, groundballing magic to post a 1.11 ERA since June 20, that following a rookie season in which he set a major league record for shutout innings to start a career (39) and finished with a 1.06 ERA. What Ziegler's doing works.
10) Brandon Lyon, Tigers
Part of the miserable package the Diamondbacks got for Curt Schilling after the 2003 season, Lyon is hardly an intimidating pitcher given his career 5.7 K/9, 4.27 ERA and 1.38 WHIP, but the former Arizona closer just keeps finding himself in late-inning situations. His success thus far this year is entirely due to luck. He's walking more men than ever before, but getting by due to his opponents hitting an absurdly low .221 on balls in play. Still, it's hard to argue with the results. Since the calendar flipped to June, he's posted a 0.77 ERA, 0.8 WHIP, a solid (especially for him) 2.9 K/BB and hasn't allowed a single home run.