Clayton Kershaw’s Disappearing Fastball
Clayton Kershaw is nearing the end of another very good season. For the third straight year, the left-hander will fail to record 30 starts or 180 innings, but his 3.05 FIP and 2.51 ERA, his 25% strikeout rate and 4% walk rate make him one of the 10-15 best starters in the game by rate — and still a top-25 pitcher after accounting for volume. That’s not quite the 2011-15, Cy Young-level Kershaw who averaged more than 7.0 WAR per season, but it’s still good enough that he’ll likely opt out of his deal with the Dodgers in favor of entering free agency.
As to why Kershaw is now only “really good” instead of “Death Star-level dominant,” the easy culprits are age and health. He’s 30 years old now and has spent time on the disabled list due to back problems in each of the last three seasons. Perhaps directly related to those issues has been the loss of velocity on the lefty’s fastball. The graph below shows average velocity by season and includes his slider for reference.

For a decade, Kershaw sat at roughly 94 mph with his fastball. Last year, he averaged 93. This season, that figure is closer to 91. In the meantime, Kershaw has slowly modified his slider to increase its velocity into the 87- to 88-mph range we see today. (If you want to read more about the evolution of that pitch, re-visit Jeff Sullivan’s post on the matter from back in 2014.) The point here is that the slider, while perhaps experiencing a bit of a dip relative to last year, has exhibited pretty much the same velocity this season as the past few, while the fastball has slowed down significantly. The slider has been a pretty consistently very good pitch since 2014, with whiff rates in the mid-20% range and swings on half of pitches outside the zone. The whiff rate is down to 15% and swings outside the zone are closer to 40%, but the pitch is dropping a bit lower and inducing grounders on 66% of batted balls. Due to a high infield-fly rate, only 9.3% of batted balls are flies that leave the infield. The result for Kershaw on the slider has been a 47 wRC+ consistent with his career numbers.



