What’s wrong with Kelvin Herrera?

There were any number of question marks coming into the 2017 season, but if there was a certainty column, Kelvin Herrera was one of the few Royals occupying it. Things have not gone as planned. Thus far, Herrera has a 5.13 ERA in 27 appearances, far and away his worst career mark. He has already given up seven home runs, just two shy of his career high set back in 2013. I’m sure you remember that season and/or the season long Twitter meltdown that went with it.

These numbers sting a little more when we remember how good Herrera was last season, when he set career highs in strikeouts, K/9, FIP, xFIP and fWAR. Much of that was due to the shiny new slider that he debuted during the Royals 2015 playoff run. I have a long history of terrible player evaluations, but I was pretty certain that Herrera would end hitters in 2016 because of that slider. This gif still gives me warm and fuzzies.

Unfortunately, that pitch has played a major role in his struggles. Last season, hitters hit .133 off of his slider and slugged just .200. This season, opposing batter are hitting .346 and slugging .731 against the pitch. Three of the seven home runs he has given up have been via the slider, despite throwing the pitch only 17% of the time.

The change really is surprising. His slider was absolutely dominant in 2016 and was arguably his best pitch. It shouldn’t be surprising that he has tried to build off last season’s success by throwing it more in 2017. Herrera threw his slider just 90 times in his 72 innings last season. This season, he has already thrown it 97 times in 26.1 innings.

So what the heck happened? Why did his slider go from unhittable to Jonathan Sanchez, seemingly overnight?

Well, one potential factor has been his inability to command his slider. And when I say his inability to command, I don’t mean inability to throw strikes. In fact, one could argue that his slider is spending too much time in the zone.

Here is the heat map for Herrera’s slider in 2016:

As opposed to his 2017 map:

In 2016, 44% of Herrera’s sliders were down, away and out of the strike zone to right handed hitters (or low and in to left-handers). That number is six percentage points lower in 2017, at 38%. And those sliders that are staying in the zone aren’t exactly good pitches.

He has thrown more sliders middle-middle (8) than he has any other spot in the strike zone. In 2016, Herrera only had two spots in the strike zone where he threw more than five sliders. One of those was down-and-away. This season, there are four spots like that, two of which are in the middle of the zone. That doesn’t seem like a winning solution. In fact, it is very much not a winning solution. Hitters are 7-13 on middle zone sliders with a .556 SLG%.

For reference, opposing batters mustered just a single hit on Herrera sliders thrown for strikes. Not in middle zones or inner zones. That is the entire strike zone.

And it only gets worse. Herrera’s slider isn’t the only problem. Opposing batters are also hitting .296 with a .577 SLG% against Herrera’s fastball/sinker. This one is a little more puzzling. He is getting the same number of swings and misses, is throwing just as hard, and his 2016 zone profile doesn’t look a whole lot different than the 2017 version. The only real difference is that hitters are smashing balls that they didn’t smash last year.

Notice all the SLG% on this map that are higher than 1.000. That’s not ideal.

There is still hope, though, as there always is with pitchers. He isn’t losing velocity, is still getting swings and misses and is still striking out 25% of the batters he faces. And his changeup is still working fine.

It also doesn’t hurt that lately the Royals are hitting every baseball thrown their way, temporarily taking pressure off of a bullpen that has been in need of a breather for five years.

We have seen Herrera struggle before. Hopefully this is just a struggle and not the beginning of his decline. The contributing factors seem to suggest that it is the former.