What's wrong with Bronson Arroyo?

Somebody needs to FIX IT! Image by Brent and MariLynn via Flickr

Arroyo hasn't been good this year. He's 8-7, but has a 5.69 ERA, a 6.03 FIP, a 5.15 xFIP, a 6.79 tRA, and a 5.59 tRA*. None of those is encouraging.

Last night, he said this:

“That’s the way it goes, man,” Arroyo said. “There’s nothing to say as far as why I’m giving up so many homers. I’ve always given up a decent amount. Physically, I feel good. I feel strong. I’m able to throw 90 if I need it.

“I’m in one of those ruts where I can’t get over the hump, get on the plus side, winning 3-0, 4-1. I’ve been chasing it the last couple times. All you can do is take the ball every fifth day and try to dig yourself out of it.”

The following all comes from his fangraphs page.

His home run rate is up a fair bit this season:

Predictably, his HR/FB rate is as well, and that's usually thought to fluctuate randomly. Even better, his ground ball rate is actually up this year. This is why his xFIP is so much better than his FIP. So that's good news.

What's bothering me about Arroyo is his strikeout rate. In particular, look at this graph:
What I'm seeing here is a steady decline in his k/9 rates from mid-last season until now. His k/9 rate this year is its lowest since 2005 when with the Red Sox, and to me is looking pretty scary. I've long believed that Bronson's bellweather stat is his strikeout rate, so color me concerned.

I'm no scout, so I can't give you a precise cause. But let's play a bit: his fastball run value has taken a huge hit this year, and appears to be where the problem lies among his major pitches from the pitch value data. But his fastball velocity, as he said, is essentially unchanged vs last year. And his fastball pitchf/x movement looks similar (maybe a slight drop in vertical movement, but not as large as 2007 vs 2008). Run values on his curve ball and change are actually improved this year, and are mostly unchanged on his slider, so those pitches look fine.

His walk rate is up this year. So, here's a hypothesis: Arroyo's not spotting his fastball this season, and so he can't use it to properly set up his breaking slop as he usually does. And he's behind in the count more than usual, causing him to give better pitches to hit. I can't do my own pitchf/x at this point, but would someone like to test this who can assess strike zones? Maybe compare balls vs strikes on all 3-1 and 3-2 counts in 2009 vs. 2008 in which he threw a fastball? I've gone as far as I can go.