Below is a chart that looks at how often Rays starting pitchers throw sliders since Andrew Friedman took over the front office in 2006. In Friedman’s second season (2007), the Rays starting pitchers were third in the American League with sliders representing 15.4% of the pitches thrown. Since then, the frequency of sliders has quickly fallen, to the point where it was non-existent in 2012* (AL Rank is in parentheses).
This trend has not necessarily been intentional as the Rays traded Scott Kazmir, David Price stopped throwing his slider, and the Rays were not afraid to trade for Chris Archer and his slider which is waiting in the wings. But it could be another reason why the Rays went out and took a flyer on Roberto Hernandez. For his career, 12.8% of his pitches have been sliders. And in his last two full seasons, it was 18.3% and 13.3%.
Certainly the Rays are not going to add a slider to the rotation just for the sake of adding a slider. But if the flyer works, and Hernandez proves to be a better option than Jeff Niemann, it would be nice to be able to give opposing hitters a different look than the more common fastball-curve-changeup arsenals seen in the Rays rotation.
* Jeff Niemann will throw 8-10 sliders per start. And Matt Moore does throw a curveball that looks a lot like a slider.