Adam Dunn’s Fastball Problem

The Chicago White Sox ponied up $56 million this past winter to secure Adam Dunn’s thunderous bat for four seasons. The move looked like a winner for fantasy owners. While Dunn would transition to the American League, he would get to take his hacks at U.S. Cellular Field, which increases home run production for lefty hitters by 23 percent. When you consider that Dunn deposited 38 balls in the bleachers in both 2009 and 2010 despite playing home games at Nationals Park (94 Home Run Park Factor for lefties), another 40-plus homer reason in 2011 seemed within reach. Dan Szymborski’s ZiPS projection system called for Dunn to blast 39 home runs, with an Isolated Power of nearly .290.

It’s now early June, and Dunn has taken a slow trot around the bases just five times. His .145 ISO is just slightly above the .138 major league average this season. The 31 year-old did have an emergency appendectomy in early April, and his performance did trend in the right direction in May after a ghastly opening month. But even so, Dunn — whose bat has been 30 percent better than average during the course of his career, once park and league factors are accounted for — has been 15 percent below average offensively this season. The main reason for Dunn’s downturn at the plate is that he isn’t crushing fastballs near as much as in years past.

Since 2002, Dunn has performed +1.64 runs better per 100 fastballs seen than the average hitter. That places him 10th among all MLB batters over that time frame. With the White Sox, however, Dunn’s runs/100 value against fastballs is -0.75. He had never posted a negative run value against heaters prior to this year, but the slugger is handling fastballs about as well as Brendan Ryan and Brandon Inge in 2011. Pitchers have taken notice: they’re throwing Dunn fastballs over 67% of the time this season, compared to a career rate of 59%.

There’s generally an inverse relationship between a batter’s power and the percentage of fastballs that he sees — the more pop he possesses, the fewer fastballs he gets. If you take a gander at the qualified batters with the highest rate of fastballs seen, you’ll note that Dunn’s name sticks out amongst a group of hitters who rarely do extra-base damage: other players in the top 10 in terms of fastballs seen include Chris Getz, Juan Pierre, Sam Fuld, Jonathan Herrera, Jamey Carroll, Denard Span, Brett Gardner, Ichiro Suzuki and Brett Gardner.

So, why have opponents been so willing to challenge Dunn this season? He’s having a harder time making contact against fastballs, and when he does put the bat on the ball, he’s not inflicting as much pain for pitchers.

Over the 2009 and 2010 seasons, Dunn whiffed at 9.1% of the fastballs thrown his direction (whiffs out of total pitches seen). While that was well north of the six percent big league average, Big Donkey made up for it by pummeling the pitches that he did get a hold of: his slugging percentage on contact against fastballs was .904. The major league average, by contrast, is a bit above .530.

In 2011, Dunn’s whiff rate against fastballs has climbed to 10.6%. At the same time, his slugging percentage on contact versus fastballs is a much milder .562. He has popped up 17% of the fastballs that he has put in play so far, compared to 12% the previous two years and the 9-10% MLB average.

Dunn’s tepid hitting has led to some owners cutting ties with him entirely — his ownership rate in Yahoo leagues is down to 89 percent. But, as lousy as Dunn’s season has been to this point, it’s hard to believe that he has suddenly lost the bat speed and power that have made him an elite fastball hitter over the years. His power has been way down to this point, but the sample size involved (213 plate appearances) falls well short of the 550 trips to the plate needed for a change in ISO to become meaningful. ZiPS still projects Dunn to hit 22 homers during the rest of the 2011 season, with his ISO rising to .265.

If you own Dunn, don’t ditch him now. While his fastball problem is somewhat concerning, there’s a good chance that he’ll start turning on heaters more often in the months to come.

Pitch F/X data from Joe Lefkowitz’s site.