The Diamondbacks signed Bronson Arroyo to a two year, $23.5 million contract on Friday, and predictably, response from the statistical community wasn’t that positive, especially with Paul Maholm signing for just $1.5 million in guaranteed money with the Dodgers earlier in the afternoon. Jeff already wrote up the pros and cons of Bronson Arroyo’s deal this morning, which I’m essentially in agreement with, so feel free to read that if you’re just looking for a summary of the deal. However, there’s another aspect to Arroyo that I think is worth mentioning, especially considering that he’s changing divisions.
Arroyo has really large platoon splits; some of the largest in the game, in fact. Over the last three years, he’s held RHBs to a .290 wOBA, much better than the league average, but LHBs have put up a .374 wOBA against him, making him one of the worst regular starting pitchers in baseball against left-handed bats. Among right-handed pitchers who have faced least 1,500 total batters over the last three years, only Jason Marquis (.389 wOBA) has been worse against left-handed bats. Including LHPs (which brings in the disastrous Ricky Romero), Arroyo ranks 106th out of 108 starters in wOBA vs LHBs since the start of the 2011 season. It’s a pretty big flaw.
Now, it’s not that uncommon for a pitcher to strongly favor pitching to same-handed hitters, and he’s hardly the only starter with a big platoon split. Gavin Floyd and Justin Masterson actually have bigger ratios between their wOBAs against lefties and righties, and Max Scherzer, Rick Porcello, and Lance Lynn aren’t far behind. These are all pretty solid pitchers — Scherzer obviously is more than just solid — and shows that a big platoon split doesn’t ruin a RHPs chance of sticking in the rotation. After all, you only have a big split as a Major Leaguer if you’re particularly good against same-handed hitters, so we’re selecting for pitchers who are strong against one side, giving them the ability to get some hitters out with a lot of frequency.
But this also leaves them vulnerable against teams who can stack the deck with lefties, and pitchers who run big splits tend to face a lower percentage of same-handed hitters than pitchers with a more even split. For reference, here are the five RHPs with the biggest platoon ratio over the last three years, with their batters faced totals included as well.
Notice Arroyo’s Platoon%, relative to the other four? Masterson has faced nearly 60% lefties, and Floyd and the two Tigers are both over 55%. Arroyo, though, is at just 50%, as he’s faced an almost identical number of right-handed and left-handed bats since the start of the 2011 season.
A big part of that is that he’s the only National League starter in that group. An AL manager is never going to put a right-handed DH in the line-up against these guys if they can help it, so there’s basically an extra left-handed bat in the line-up against the AL platoon guys every single time out. It’s simply easier for AL managers to exploit platoon splits than it is for NL managers, which is one of the reasons why pitchers like Arroyo have more success in the NL than the AL.
But it’s not just the National League factor pushing Arroyo’s ratio of left-handed hitters down. It’s also the National League Central factor, and maybe even more specifically, a Cincinnati Reds factor. The Reds themselves had a decent amount of left-handed hitting last year, ranking 3rd in the NL in PAs from LHBs, but of course, Arroyo didn’t have to pitch against his own team, and the rest of the NL Central teams simply didn’t have much in the way of left-handed hitting.
The Cubs and Cardinals ranked 6th and 7th in the NL in PAs from LHBs, while the Pirates ranked 13th and the Brewers ranked 15th. The Brewers, in fact, were on an island to themselves in terms of right-handed slant, as every other NL team had at least 2,000 PAs from LHBs in 2013, while the Brewers had just 1,690. The Phillies and Giants sent nearly twice as many left-handed hitters to the plate last year as the Brewers did.
This wasn’t just a one year fluke, either. The non-Reds NL Central teams ranked 6th/9th/10th/11th/15th in LHB PAs in 2012, and 7th/9th/12th/13th/16th in 2011. For whatever reason, the NL Central has just not had many left-handed hitters over the last few years, with nearly every team skewing to the right side of the plate. So perhaps it shouldn’t be a huge surprise that other high Platoon Ratio/Low Platoon% pitchers happen to pitch for NL Central teams as well. Charlie Morton (PIT) has had a whopping 1.44 Platoon Ratio, but just 46% of his batters faced have been LHBs. Lance Lynn (STL) posted a 1.26 Platoon Ratio, but only faced 46% LHBs over the last three years. Shaun Marcum (MIL, mostly) put up a 1.24 Platoon Ratio, but faced just 49% LHBs from 2011-2013. Jeff Samardzija (CHC): 1.14 Platoon Ratio, 47% Platoon%.
Outside of NL Central hurlers, we just don’t really see examples of righties who struggle against lefties getting to face more righties than lefties. Especially in the AL, these pitchers end up facing 55-60% LHBs, and even in the NL West and East, the number is more regularly in the 52-53% range. And unfortunately for Bronson Arroyo, he doesn’t get to take the NL Central’s distribution of hitters with him to Arizona.
How big a deal is a few percentage points in platoon distribution? Well, let’s use Arroyo as an example. If you just take his three year wOBA against numbers (.290/.372), here’s what his total wOBA allowed would look like based on different platoon ratio distributions.
You take the same pitcher with the same skills and move him from a 50/50 to a 40/60 distribution, and his wOBA allowed goes up eight points just from the additional number of left-handers he’d have to face. Eight points of wOBA might not seem like a big deal, but over 850 batters faced, that adds up to about an extra six runs allowed per year. Over 200 innings, that’s roughly equivalent to 20 points of ERA.
Now, Arroyo almost certainly won’t face 60% left-handed hitters in Arizona, since it’s still an NL team and he won’t have to face DH-filled line-ups too often, but this is one of those subtle things that suggests that Arroyo’s change of location might make him a little worse off than even the modest projections already suggest, and perhaps more importantly, this flaw significantly limits Arroyo’s usefulness in October. While there’s an argument to be made that regular season totals understate the importance of frontline pitchers and ace relievers in the playoffs — which is likely one one the reasons why teams pay premiums to acquire these types of players — starters with big platoon splits are less valuable in October than they are in the regular season.
If Arizona happens to make the postseason, any team facing them in the NLDS or NLCS will have the ability to stack its roster in such a way as to force Arroyo to face a bunch of left-handed bats, and if Arizona makes it to the World Series, starting him in an AL park is basically a no-go, because otherwise he’d be tasked with facing yet another left-handed bat in the form of the DH. In other words, the Diamondbacks paid Arroyo $23.5 million to help them make the playoffs in the next two years, but if they get there, he probably won’t be able to actually help them much in the postseason, given that his success is pretty dependent on facing line-ups filled with right-handed hitters.
This isn’t just a warning for the Diamondbacks. This is relevant for any team trading for Samardzija, Lynn, or any other right-handed pitcher in the NL Central. It’s not a huge factor and shouldn’t be the reason why you don’t complete a trade for a hurler from that division, but it’s worth keeping in mind that the RHPs in the NL Central are probably not quite as good as they look.
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