From 2004-08, any conversation about who was the best pitcher in baseball had to include Jake Peavy. During that span he made two All-star games (starting one), was the ace of team USA in the inaugural World Baseball Classic, lead the San Diego Padres to two division titles, and was the NL Cy Young winner (by unanimous vote) in 2007. Over the course of those five seasons his numbers were very impressive:

Stat Rank** ERA 2.95 2nd FIP 3.18 2nd xFIP 3.43 7th K/9 9.38 2nd K/BB 3.46 10th fWAR 21.8 9th

**Note: Rank is among pitchers with a min. of 700 IP over those five seasons

Peavy has helped lead his current team, the Chicago White Sox, to an unexpectedly hot start, as David Fung pointed out in an infographic, last week. Due in large part to back-to-back complete games against Oakland and Boston, Peavy currently supports the third-highest fWAR (1.5) among Major League starters (behind only Justin Verlander and Jered Weaver, who each have one more start than Peavy) and a ridiculous 1.67 ERA, backed by his 2.05 FIP. But why are people so surprised by Peavy's hot start? He used to be one of baseball's best starters, as shown by the numbers above, thus what's all the fuss about? Because Peavy just hasn't been himself in a long time.

I'm sure it will surprise most people who follow the sport that Peavy will turn just 31 at the end of May. It feels like he's been in the league forever (debuting as a 21-year old), but the last couple of seasons have made him seem so much older. The reason for this warped perception of Peavy's age and the reason why his start is so surprising are one in the same. The injury bug, of course. Peavy's Injury history on his Baseball Prospectus' player card is about as long as the Encyclopedia Britannica. He has appeared on the DL six times over the course of his career, five of those stints occurred between 2008 and 2011. From ’08 and ’11 he missed a combined 248 games, roughly 50 starts (12.5 starts on average per season). Peavy just wasn't able to stay on the mound during those seasons (especially from '09 to '11), but so far in 2012 he looks healthy and effective. Thus, I beg to question whether the Peavy we've seen in his first five starts of 2012 is the same guy we saw dominate hitters from '04 to '08?

The simple answer is no, he is not. RJ Anderson wrote an excellent article on BP this week about why Peavy is a different hurler than the Peavy of yesteryear. Anderson brings up two very distinct points as to why this is a much different Peavy than the one we saw in the past. Anderson points out that Peavy is now locating a large percentage of his fastballs up and away to righties, and up and in to lefties. He attributes the high fastballs to the massive amount of fly balls Peavy has been giving up. He currently holds the lowest ground ball percentage (24.5%) among qualified starters in baseball. Peavy has never been a ground ball pitcher, but his current percentage is uncharacteristically low. Anderson notes that while this new approach to locating his fastballs a different look for Peavy it has been a very effective approach. Anderson also points out that Peavy's strikeouts aren't has high as they were in during his "vintage" period. Strikeout rates over nine were the norm for Peavy during the '04-'08 stretch, but his current K-rate is just under eight (7.88). Anderson is not trying to say that Peavy's current rate of 7.88 is low, just that "vintage Peavy" struck out a ton of batters, and the Peavy we've been seeing in 2012 is not going to do that.

One aspect of Peavy's great start that Anderson did not mention was his low walk-rate. Peavy's career BB/9 coming into 2012 was 2.84, and over his dominant five seasons that I keep discussing, his walk rate was 2.71. Yet thus far in 2012, he has had no trouble with issuing free passes, walking just 1.19 batters per nine innings, eighth-best amongst qualified starters. While he hasn't faced the most patient lineups in baseball, the five teams he's faced currently rank in the bottom half of BB% to start the year, that factor can't be the sole reason for the fact that his walk rate is uncharacteristically low. His low walk-rate and fairly high amount of K's, has resulted in a 6.6 K/BB rate, fourth-best among qualified starters. Most would argue that a reduction in slight reduction in strikeouts coupled with a serious reduction in walks, resulting in a high K/BB rate, makes for a more effective, and thus productive pitcher.

So Peavy has looked like a different pitcher than in years past, but different has not meant bad or worse as he has looked like a very good starting pitcher. Is there any chance Peavy can keep these elite ace-esque numbers up? Or are there serious red flags in Peavy's numbers that will inevitably cause them to regress?

The Red Flags

-Home runs. Peavy has only given up one home run, thus far in 2012, which doesn't sound that ridiculous after only five starts. However, he sports baseball's highest FB% (57.1%) by over 5%, as Anderson pointed out these fly balls most likely can be attributed to the high fastballs that Peavy keeps throwing. Peavy's current 1.8% HR/FB rate is just not sustainable, his career HR/FB% was 9.5% coming into 2012. I think there is a good chance if Peavy keeps giving up this many fly balls that his rate will be below his career average, because of the sheer numbers of fly balls that will he'll be inducing. His HR/FB rate will in all likelihood even out to somewhere between 7 and 8 %, which would still be a serious regression. This home run regression accounts for the gap between Peavy's current FIP (2.05) and xFIP (3.74). The main issue with Peavy becoming an extreme fly ball pitcher is his home park, US Cellular Field.

Based on ESPN's Park Factors over the past five years, US Cellular Field has ranked in the top-5 of homer-friendly ball parks, ranking as the most home run friendly in 2010. Being an extreme fly-ball pitcher would probably have been a better strategy when Peavy was still pitching for San Diego, as Petco Park is a historically home run/pitcher friendly place to play. A FB rate above 50% may make it seem like Peavy's playing with fire, in his current home park; however, Peavy's HR/9 rates tell a slightly different story. Coming into 2012, Peavy's career HR/9 as a Padre was 0.905, while his HR-rate as a White Sock was exactly the same, 0.905. To delve further into those numbers, coming into this season Peavy's career rate at Petco was 0.63, while on the road as a Padre it was 1.23. As a White Sock in US Cellular his career rate was 0.82***, and on the road was 1.00. If Peavy is able to keep his home run rate under 1, at home, while giving up a ton of fly balls, his numbers (ERA for instance) may end up looking better than his xFIP of 3.71, which is still a pretty good number as it is.

***Note--The lone home run Peavy gave up thus far in 2012, came at home, but his career rate in US Cellular decreased despite that home run, and now resides at 0.75.

-BABIP. Batting average on balls in play seems to be the stat that everyone in the baseball world wants to talk about at the moment. Jeremy Hellickson's extremely low BABIP, how much control a pitcher actually has over his BABIP, how big a factor team defense plays into BABIP, and many more, are all being discussed across baseball's mainstream sites, and of course, the blogosphere. Peavy holds a career BABIP that is slightly lower than average (.284), but not low enough that any significant difference can be assumed between it and the league average. Peavy's current BABIP (.206) would cause a lot of people to cringe, because that number is clearly not sustainable. ZIPS projects his BABIP to be .317 the rest of the way, which just happened to be his BABIP in 2011, and thus projects his BABIP to regress back to about league average (.294). I expect Peavy's BABIP to rise (and his ERA as well); however, I don't expect it to rise just for Peavy, but instead for the White Sox team as a whole. Last year, Chicago had the second-highest staff BABIP (.303), yet thus far in 2012 they have the second-lowest (.251). Chicago is not a great defensive team, by any stretch of the imagination, which is why their current BABIP and Peavy's will inevitably regress as the season goes on.

-LOB% (Left on-base Percentage). Peavy hasn't let a lot of runners reach base this year, as noted above by his low walk-rate and BABIP. The runners that have reached base, for the most part, Peavy has stranded; his current LOB% (78.1) seems like another possible candidate to regress as the season goes forward. His career LOB%’s with the White Sox is below 70%, which would lead most to expect a regression in this area. However, Peavy has had seasons, during that '04-'08 time period, where his LOB% finished above 78.1%. Thus, I don't think it would be that bold to predict that his LOB% wouldn't regress a great deal, if at all. A 78.1 LOB% isn't impossible to sustain over the course of an entire season, contrary to a .206 BABIP or a 1.8% HR/FB, as 17 qualified starters in 2011 finished with LOB%'s at or above 78.1%. Peavy has been described many times as a bulldog, he's gritty and can bare down on hitters, especially with runners on base; thus, I think this "red flag" is the least concerning of the three.

It is pretty certain that more balls are going to fall in play against Peavy, and that more of the fly balls he gives up end up in the seats. But if he can keep the walks down and stay healthy (which is the big IFin Peavy's case) then there's a legitmate chance that he could be one of baseball's best again in 2012. His current peripheral stats are evidence to this statement and my final point. Peavy's xFIP and SIERA are 3.74 and 3.42, respectively. I'll just end this article by putting those numbers in perspective. Pitchers who had xFIP's higher than 3.74 in 2011 include: Weaver (of no-hit fame), Daniel Hudson, Derek Holland, Ricky Romero, Matt Cain and Jordan Zimmermann. Pitchers who posted a SIERA above 3.42 in 2011 include: Tim Lincecum, Josh Beckett, Ian Kennedy, CJ Wilson, and Mat Latos

All statistics came from Fangraphs and Baseball Prospectus.