Where Did Sean Manaea’s Strikeouts Go?

I was a big fan of Sean Manaea heading into the season, believing that his above average SwStk% and superb minor league strikeout rates, would lead to a strikeout rate surge this year. Through April, my optimism was paying off, as he had struck out 23.6% of the batters he faced and posted a microscopic 1.03 ERA. Ever since that sparkling month, his ability to punch out batters has completely eroded. Why?

There are many reasons a pitcher’s strikeout rate could collapse. Let’s investigate several of those possible reasons, which include:

-A loss of velocity
-An unfavorable change in pitch mix
-A decline in the whiffiness of a particular pitch

Loss of Velocity

We’ll begin by comparing his velocities from 2017 to April 2018 to Post-April 2018.

Manaea’s fastball and changeup velocity is down ever so slightly from last year, but clearly not by enough to explain anything. And if we’re just comparing his April to Post-April of this season, he has kept his velocity essentially stable, with a slight uptick on his two primary pitches. Of course, pitcher velocities do creep up throughout the year on average, so we would have expected this.

So far, his velocity doesn’t appear to be the answer.

Unfavorable Change in Pitch Mix

Now let’s go over his pitch mix over the three periods to see if perhaps this might provide an explanation.

Okay, so now we see some changes. That does make sense, of course, as no pitcher perfectly maintains his pitch frequencies every season, or even during the season. In April, he threw about the same rate of fastballs as in 2017, but that rate jumped after the first month. Fastballs generate the lowest strikeout rate, so all else equal, you would expect that more fastball will result in fewer strikeouts.

In addition, Manaea’s usage of his changeup has dipped and he’s been throwing it even less than he did in April, which already represented a significant decline last year. That’s odd, because his changeup has consistently induced swings and misses, yielding mid-to-high teen SwStk% marks the last two seasons. He’s tried to offset the loss of changeup whiffs by upping the usage of his slider. The pitch actually generated a higher SwStk% than the changeup in 2017, but is slightly below this year.

So unlike his velocity, the pitch mix analysis did tell us something. Throwing more fastballs is typically bad for strikeouts, so that could provide a small piece of the strikeout rate decline explanation.

Decline in the Whiffiness of a Particular Pitch

Since we don’t have the ability to filter a pitcher’s SwStk% by pitch by date, I’ll resort to comparing Manaea’s SwStk% mark on each pitch from 2017 to 2018.

And there you have it. So not only is he throwing his worst swing and miss pitch more often, but his two non-fastballs have generated fewer whiffs. Both still remain quite respectable in that department, but haven’t been nearly as whifftastic as last year. We noted that the velocities have remained stable, so it’s perhaps a matter of movement or location.

Still sporting a solid 3.50 ERA, he looks like an obvious sell candidate. With a SIERA nearly a full run higher thanks to a .240 BABIP, and a lowly strikeout rate that has sat in the teens every month since April, he could easily deliver negative value the rest of the way.