Heading into the 2010 season, it was this analyst's opinion that Jake Peavy's days as an ace were over, between a drop in his velocity and a switch to the American League. Even with that though, he was still ranked as a four-star starter, along with the likes of Clay Buchholz, Max Scherzer, and (a healthy) Brett Anderson. Things have not worked out like that though, as Peavy currently sports a 5.90 ERA over 76 1/3 innings pitched.
SIERA has Peavy at 4.00, which would put him at #19 in the majors amongst pitchers with as many innings as him (and also much closer to where his fantasy rankings felt he would be). SIERA is optimistic because Peavy is whiffing 7.7 batters per nine (above the league average) while walking 3.1 per nine (also better than the league average), and he keeps the ball on the ground often enough that SIERA doesn't think his 1.3 HR/9 rate is going to stick.
Peavy is pretty close to the league average as far as G/F ratio is concerned—he was at 1.1 for each season from 2007-2009, and he's currently at 1.0 this year. Homers have never been much of a sore spot for Peavy either, as he's been around the league average or below it for much of his career. He's had some outside influences help him in those regards though—Petco Park was his home stadium from 2004 until he was dealt last year.
2009 data isn't of much use to use, because Peavy logged about 40 innings on the road (some of which were in the AL) but if you look at 2008, he had a massive split in his home run rates. At home, he gave up four total bombs over 98 innings, or just under 0.4 per nine. On the road, he allowed 13 over 75 2/3 innings, or just over 1.5 per nine. It's just one year of data, but it was something to consider when thinking about his new digs—he's at US Cellular now, which is known for the long ball in the same way Petco is known to suppress it.
That's why the automatic assumption this year with Peavy was that his home park was hurting him in the same way Petco used to help him, but a quick look at his splits shows that is not the case as of yet. Peavy has given up 11 homers on the season, but just three of those in Chicago. The other eight have all been on the road, and in 1 2/3 innings more time.
The pessimist in all of us may want to say that the samples are too small and that Peavy may end up taken deep at home on a consistent basis given more exposure, so even if his road numbers fall back, he'll be hard-pressed to match his SIERA. I'm comfortable thinking this is one of those times where we should trust the run-estimator though, but with the caveat that Peavy may perform a bit worse at home in the long run, even as his road numbers improve. Putting him down for an ERA of 4.2 or so going forward seems safe—it's not the best ERA, but he doesn't put a ton of runners on and he still posts good (though falling compared to his NL rates) strikeout totals. He's not an ace by any means, but he's not a replacement level hurler either.
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