Trevor Rosenthal began the 2016-17 offseason preparing to stretch out as a starter. Consequently, I’d speculate, he spent the offseason working on the pieces of his repertoire he featured as a starter in the minor leagues but put aside as a full-time reliever.
Specifically, those discarded pitches included a curveball, which Baseball America gave a 50 grade, and a slider, which FanGraphs called “better than average” in Rosenthal’s 2013 profile. While we haven’t seen a curve this year, we have seen a revamped slider. Even back in spring, Jenifer Langosch reported the slider was a developmental focus for Rosenthal.
We got our first look regular season look at the new slider back against the Washington Nationals. After that game, I commented on his impressive understanding and use of sequencing for the slider. Joe Schwarz declared “Trevor Rosenthal isn’t fair to opposing hitters.”
At that time, however, Rosenthal had only thrown two sliders. Still, I thought it was telling that in the three pitches he deviated from the fastball, he went to the slider twice. Consider his historical pitch usage rates: from 2013-2016, he threw changeups 15% of the time and breaking balls 7% of the time. In 2017, that usage has flipped: Rosenthal is throwing the slider 14.5% of the time and the changeup on 6.8% of his pitches. On Thursday night, it looked like this:
— smusial (@SimulacruMusial) April 28, 2017
Those pitches are well-located and downright nasty. Rosenthal’s slider rates as his best pitch this season measuring by FanGraphs standardized linear weight pitch values. This stat measures the runs above or below average that the pitcher produced on a per 100 pitch basis, and a positive number is always better for the player in question. While the sample size this season is too small to generate any predictive conclusions, it confirms that Trevor Rosenthal has thrown a better slider so far.
Rosenthal has always located the slider well, as evidenced by his slider location heatmap from 2012 to 2016 compared to the 2017 heatmap. He’s throwing it at 89 mph, which is 4 mph up from last season but in line with its velocity from 2013 to 2015. Additionally, he’s getting the same horizontal break than he ever has.
The difference this year is spin rate and vertical movement. While he’s getting less drop than he did in 2016 when he threw the slower slider, he’s getting 3 inches more than he did using the hard slider from 2013-2015.
So, how do you get three more inches of vertical movement without sacrificing velocity or horizontal movement? You add spin. And that’s exactly what Trevor Rosenthal has done.
Spin rates are available via Baseball Savant starting with 2015. As you can see, Trevor Rosenthal has generated an increase of more than 500 rpm compared to last year and an increase of nearly 400 rpm compared to 2015. The total spin rate on his slider is creeping up to that of his fastball, which should make his slider more deceptive.
Additionally, the changes in spin look completely intentional. While his fastball spin was relatively stable from year-to-year, both his slider and changeup spin have changed drastically. Further, each moved in the positive directions for each respective pitch (more on the changeup another time).
How drastic is the increase in slider spin rate? Looking at all 234 pitchers who threw 10+ sliders in 2016 and 2017, Trevor Rosenthal’s 512 rpm increase is the largest.
An increase in slider spin rate is almost always a good thing. Early research from Jonah Pemstein at FanGraphs suggested more slider spin led to less swings, a lower contact rate, less line drives, more pop-ups, and a lower HR/FB rate. For swinging strike rate, it’s best to be either on the extreme high or extreme low end. Rosenthal’s average slider spin rate currently ranks just outside the top third and is obviously trending in the right direction.
The slider is arguably his best offspeed pitch already and, perhaps more importantly, gives him a third plus pitch to attack hitters with. It makes Rosenthal less predictable and adds an element to his repertoire that he’s been missing. While we probably will never see Trevor Rosenthal as an MLB starter, there was still a positive outcome from this offseason’s attempt.