A closer look at the approach of St. Louis Cardinals closer Trevor Rosenthal

By fWAR, St. Louis Cardinals closer Trevor Rosenthal (5.5) has been the eighth most valuable relief pitcher since the start of the 2013 season, behind names like Aroldis Chapman (7.0), Kenley Jansen (6.3), Mark Melancon (6.1), Craig Kimbrel (6.0), etc. Just as I did last offseason (almost one year ago exactly), I decided to take a closer look at Rosenthal's approach—from frequency to location to sequencing—in hopes of uncovering some useful nuggets of information regarding the All-Star.

2015 PitchF/x basics

Remember: Regarding horizontal movement in right-handed pitchers, a negative value means arm-side movement, whereas a positive value means glove-side movement.

On the surface (forget about the frequency column for the moment), it appears as if Rosenthal possesses a complex repertoire (with the ability to throw four different pitches), but up to this point in his career as a reliever, the 25-year-old righty has attained much of his success through two pitches: his fastball and changeup (90.43% of his pitches in 2015).

While Michael Wacha had a solid rookie season as a starting pitcher using primarily these two pitches, he has now embraced the fact that in order to remain consistently successful as a starter, he needs to incorporate more pitches into his repertoire, just as he has done with the cutter and curve. Yet, as a closer, who may see a hitter only two to three times per month, a repertoire centered around two pitches (especially plus to plus-plus ones like Rosenthal's) can still allow for consistent success.

Year-to-year statistics

After a ridiculously effective 2013 campaign, Rosenthal took a step back in 2014 (check the BB% column), only to return to near-2013 form last season, earning his first All-Star Game selection. Plus, while one can reasonably argue the value of the "save" statistic (yet, at the same time, "contracts can't be ignored in setting bullpen roles"), Rosenthal set the Cardinals' record last season with 48. Given that he is getting to the point in his career where his performance defines his next contract, you better believe he has his eyes set on besting his 2015 mark in 2016.

So, what happened in 2014, then? Other than the obvious spike in walk rate, there had to be something else, right?

Year-to-year performance versus right-handed batters

To be clear, I understand the inherent flaws associated with analyzing results involving small sample sizes. That said, one-year splits do not mean much with relief pitchers, but I looked at them anyway. While Rosenthal's results against left-handed batters have pretty much held steady since 2013, he experienced a blip in wOBA versus righties in 2014 (as shown in the table above. Being a fourseamer-first pitcher, I wanted

courtesy of @darenw's BaseballSavant.com

It is pretty easy to tell which heatmap stands out from the others. In 2014, the core (the dark orange/brown spot) of his fourseamers to right-handed batters was up the zone, and in 2013 and 2015, this core was down in the zone. While I didn't include the X or Y axes in the illustration, Baseball Savant provided values for these cores on their site. In 2013, the Y value was 2.25. In 2014, the Y value was 2.75, and in 2015, the Y value returned back down to 2.25. Thus, the year in which Rosenthal experienced less success against righties (2014), he was throwing his fastball up in the zone. Can I guarantee that this was the reason behind a decline in performance? No, but logically it makes some sense. Frankly, a successful Rosenthal will likely continue to live down in the zone against righties.

An appreciation of his fourseam fastball

Similar to what I did last March with Lance Lynn, I looked at the final two-pitch sequences of each strikeout recorded by Rosenthal in 2015. Why the final two pitches and not just the final one? Because many pitchers "set-up" their "put-away" pitch either by changing speeds or changing the eye level by utilizing different locations. Unsurprisingly, 93.7% of Rosenthal's strikeouts included a fastball in one of the final two pitches.

What was surprising, though, was the fact that Rosenthal finished off batters with a "fastball-fastball" combination 53.2% of the time. This helps portray just how good Rosenthal's fourseamer actually is. Even when hitters were given two chances, in a row, to hit his fourseamer, they often couldn't, leading to the majority of his strikeouts. As we know, Rosenthal ran away with the poll for "who has the best fourseamer on the Cardinals?" in December, and this provides backing to those results.

That being said, there were three outings in 2015 where Rosenthal threw nothing but fourseamers (July 2nd, September 15th, and October 13th). Given how good his changeup is, this should never happen, regardless of the quality of his fourseamer. I talked about it last offseason, and I am talking about it again, let's hope this fastball-happy mood is eliminated in the future.