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Jack Flaherty’s Evolving Slider

New York Mets v St Louis Cardinals Photo by Michael Reaves/Getty Images

Jack Flaherty is a bit of an interesting ace. Unlike most aces and successful starters, he really only throws two pitches. 27.6% of his pitches last year were sliders and 58.4% of his pitches were fastballs. This means that just 14% of his pitches were either changeups or curveballs. Additionally, he primarily throws his changeups and curveballs to left handed hitters as he threw 47 of 64 changeups and 309 of 383 curveballs against lefties. This makes sense because, traditionally, sliders are supposed to be less effective against hitters with a platoon advantage. However, this was the case for Flaherty this season as his slider was extremely effective against lefties.

There could be many reasons for this. One is the fact that his arsenal expands against lefties as he throws more curveballs and sliders against opposite-sided hitters. This could cause the hitter to worry about a variety of different pitches and allow the effectiveness of his slider to increase. However, it could also be due to the fact that Flaherty has progressively changed the shape of his slider over the course of his MLB career.

In 2017, his slider had just 2.5 inches of horizontal break. Then in 2018, his slider had 5.4 inches of horizontal break. This increased again last year when his slider averaged 6.5 inches of break. Additionally, the vertical movement on his slider dropped from 34.7 inches in 2018 to 33.3 inches last season. This trend towards increasing horizontal movement and decreasing vertical movement correlates with an increase in the effectiveness of Flaherty’s slider against left handers. In a small 2017 sample size, lefty hitters recorded a .408 wOBA against Flaherty’s sliders. This dropped to .300 in 2018 and .201 in 2019. Additionally, the whiff rate of left handers against his slider increased from 36.4% in 2017 to 41.5% in 2018 to 43.6% in 2019. It is clear that his slider has become more effective against left handed hitters each year.

This might actually be due to the shape of the slider. It has gotten flatter with more horizontal movement since Flaherty entered the league. This helps to distinguish it from his curveball which also has a lot of vertical movement. Additionally, he primarily throws curveballs to left-handed hitters so by flattening his slider a bit, he may be attempting to differentiate his two breaking balls more. This would give hitters two different looks which could allow each pitch to be more effective. If this was the goal, then it has been working.

Now that his slider has changed so much, it may be time for Flaherty to throw more curveballs. This could give him a go-to third pitch that would look much different from slider while also changing speed significantly. Flaherty’s slider averaged 84.8 mph last season while his curveball travelled at just 78.2 mph. If he could begin to throw more curveballs to hitter on both sides of the plate, then it would allow him to effectively change speeds. Additionally, since he has changed the shape on his slider, it is more unique from his curveball which could allow both pitches to be more effective.

Flaherty has a very good fastball and a very good slider, but he could potentially become a more effective pitcher if he can begin to throw more curveballs in order to give hitters a different look and keep them worried about a more drastic change of speed. Flaherty’s evolving slider has allowed for this possibility of a effective curveball which dives towards the plate instead of cutting across it. He already has a curveball that drops 56.5 inches on average, now he just needs to throw it more.