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Certain players seem to fall into the “injured or effective” category; you can’t count on their presence on an active roster, but between DL stints, they put up numbers pretty enough to make you dream about what you’ll be missing out on during their next convalescence. Rich Harden may have epitomized this type before his disastrous 2010; Ben Sheets, J.D. Drew, and Nick Johnson belong to the same genus. Another player who tantalizes fans during his all-too-infrequent stints on the field is Dodgers reliever Hong-Chih Kuo, who’s fallen into both the “injured” and “effective” buckets this season. Kuo began 2010 on the DL after suffering right elbow soreness in spring training, but pitched well enough after his late-April activation to earn a selection as MLB’s 82nd All-Star.

Kuo has allowed only 3 runs in 27 1/3 innings on the season, which equates to a shiny .99 ERA. As one might expect, that performance has been aided by a hearty helping of luck; among pitchers with over 25 innings pitched, his .226 BABIP ranks 13th-lowest , his LOB% ranks 7th-highest, and his 3.6 HR/FB% also places among the most miniscule. Kuo’s fine numbers aren’t solely a product of good fortune, however; his 2.02 SIERA reflects some solid peripherals, including the 9th-best strikeout rate of any pitcher with a minimum of 20 innings pitched (11.9 K/9).

Kuo has yet to allow a single hit to southpaws this season. Same-sided batters have gone 0-for-32 off of the Taiwanese lefty (with 3 walks), and half of those outs have come via the strikeout. Despite his pristine performance against portsiders, labeling Kuo a lefty specialist would be an injustice; he’s faced nearly twice as many righties as lefties this season, and more than held his own against them despite the platoon disadvantage.

This year’s model of Kuo differs from the one who took home the hardware as MLB’s best setup man two seasons ago. In 2008, Kuo averaged nearly 2 innings per relief appearance. That extensive usage, coupled with a rate of performance roughly akin to this year’s, allowed him to make a much greater impact than the typical reliever, but his arm wasn’t up to the task. Something of a medical miracle, Kuo underwent two Tommy John surgeries before he’d pitched his 50th professional inning, and his elbow-related issues resurfaced in the wake of his 2008 heroics. Kuo suffered left elbow inflammation in September of ’08, and elbow stiffness in spring training '09, then spent 85 days on the DL with right elbow inflammation beginning last May. In response to that breakdown, the Dodgers limited his workload; the fragile lefty averaged under an inning per appearance last season, and just over an inning per appearance in 2010.

This season, Kuo’s pitch selection actually mirrors the distribution he featured as a starter before becoming a reliever exclusively in early 2008. In 2006 and 2007, Kuo threw fastballs between 60% and 65% of the time, according to the BIS pitch-type data hosted at FanGraphs, but went to his heater just under 81% of the time out of the bullpen in 2008 and 2009. This year, whether as a result of increased confidence in the soundness of his arm, an elevated affinity for the slider, or some other factor, he’s returned to the old formula, throwing smoke just over 65% of the time. His fastball has never been faster (only Matt Thornton and Billy Wagner have exceeded Kuo’s 94.4 average fastball velocity from the left side this season), and his slider has never been more whiff-inducing. In recognition of his exploits, Joe Torre has rewarded Kuo with the second-highest leverage score (1.43) in the Dodgers’ bullpen, behind only that of closer Jonathan Broxton.

As effective as his efforts have been, Kuo’s run of performance appears more impressive by virtue of the length of time over which it’s been sustained. Remember that we’re talking about fewer than 30 innings here. Look up game logs for any starting pitcher having a quality season (and maybe even a few who aren’t), and you can find similarly impressive streaks without devoting undue strain to the search. From April 7th through April 24th, Andy Pettitte allowed 4 runs over 28 innings (1.29 ERA). From April 13th through May 5, Doug Fister allowed 3 runs over 31 innings (.87 ERA). Heck, from April 11th through May 9th, Livan Hernandez allowed a mere 5 ER over 43 1/3 IP (1.04 ERA). And those were just the first three guys to come to my mind.

In isolation, Kuo’s performance in 2010 might seem especially awe-inducing because his brief run of success has been stretched out over 2 ½ months, as well as because the 27+ innings in question happen to constitute his whole season, rather than a blip in the middle of one. Therein lies the danger of judging relievers by the same framework within which we evaluate starters; the sample sizes associated with the relief role are inherently small enough that a spot of good or bad luck can cause us to overvalue or unfairly discount their performance. Perhaps there’s no place for bullpen material in the All-Star Game, but in light of his mastery on the mound when healthy (not to mention his flair at the plate), Kuo certainly merits a mention among the cream of the middle relief crop.

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