On July 6, with the Royals tied in the 8th inning against the Blue Jays, Kelvin Herrera took the mound.

It was not a good outing.

It began in promising fashion. Herrera jumped ahead of Ezequiel Carrera with three consecutive fastballs. He finished him off with a slider that probably caught too much of the plate, but it was one the Toronto left fielder could only watch.

The next batter was Josh Donaldson. Herrera starts him with high gas that Donaldson can only wave at – a 99 mph fastball for strike one. The next pitch is a slider, down, and the Jays third baseman fouls it off. Ahead 0-2, Herrera goes for a sneaky curve low and away, out of the strike zone intended for a swing and miss, but Donaldson holds steady. The next pitch is a slider, more centrally located, but further down than any other pitch he’s seen. Swing and a miss. Two down.

Two batters, two outs. Both on strikeouts against Herrera’s slider.

As I wrote way back in March, Herrera began toying with the slider around the All-Star Break of 2015. He flashed the pitch occasionally, and it was successful. Then, in September and during the postseason, it became weaponized. It developed into a powerful, put-away pitch that left opposing batters flummoxed. Through his first 39 appearances in 2016, the slider was still doing some serious work for the Royals’ eighth inning man. He used it around 13 percent of the time with outstanding results. When batters swung at it, they missed over half the time. (A 54 percent whiff rate.) In fact, heading into July, opposing batters had only collected two base hits against his slider, both singles. That’s an .087 batting average against.

Herrera was already a good reliever. The slider elevated him to a great one.

Up steps Edwin Encarnacion. The Jays first baseman passed on a slider that pushed too far inside for ball one. Herrera misses up with a change and then down with a fastball. Behind in the count 3-0, he offers a “get me over” fastball that wasn’t in the zone but got the benefit of the doubt.

On 3-1, Herrera delivered another slider, his sixth of the inning. This one was barreled and pulled to left for a double.

Four pitches later, a pair of four-seamers were raked for a single and another double, the tie was broken.

Herrera faced one more batter, Troy Tulowitzki. Three pitches – slider, fastball, slider – and the frame was over.

Three strikeouts for Herrera, all on sliders. But, so too, was the double that ignited the rally. He wore the loss for the Royals that day.

Why rehash some random outing? Why highlight one seemingly random game out of the 162? This game is the focus because after July 6, Herrera took his slider, his most effective pitch, and put it in his back pocket for the rest of the season.

From Brooks Baseball, here’s the breakdown of Herrera’s slider usage, using the July 6 game as an endpoint.

Part of the fun of digging in the PitchF/X files is there are inconsistencies from site to site. Statcast has Herrera throwing 100 sliders up to July 6 and 16 from there to the end of the season. The raw numbers don’t match, but the variation in percentage of number of sliders thrown in relation to total pitches does.

Did you notice how Herrera finished the season?

In September alone, Herrera allowed 10 earned runs in 11 innings of work. Certainly, that skews the data in the table above, but his ERA and opponents averages was higher in July and August than in any of the first three months of the season. He wasn’t the same pitcher after that outing in Toronto. It was because he stopped using his slider.

Why? Why did he stop going to the slider?

At first I thought maybe he lost the feel for the pitch. Looking at the Statcast data, that didn’t seem to be the case.

The velocity was basically the same, but Herrera was generating more spin and thus more break on his slider from July 7 onward. We have to take this with the small sample size caveat as there were just 16 pitches, but it certainly looks as though the pitch was even nastier through to the end of the year than we saw in the first three month.

If he didn’t lose the feel for the pitch, did he lose confidence in it? That seems silly to even suggest. One slider that Encarnation hit for a double isn’t a reason to abandon something that had worked so well. Sometimes, you lose even when you throw your best pitch.

Still, something happened that made Herrera move away from a pitch that once worked so well for him in the past.

While Herrera had a decent July and August, as noted above his September was dreadful. Perhaps it was so rough because by that point hitters knew he wasn’t using his slider with the same frequency. They could basically ignore the pitch and wait to punish the curve. This was especially true of right-handed batters. According to Brooks Baseball they saw Herrera’s slider 26 percent of the time in games through July 6. After that date, right-handers saw the slider just 3 percent of the time. He had leaned on his slider as his out pitch against righties, but midway through the season shifted the emphasis to the curve. Out of sight, out of mind as they say. And for hitters from the right side, it was one less thing to worry about. The evidence supports this. Of his last 16 sliders thrown from July 7 to the end of the season, hitters swung at seven, missing only twice.

Until we hear directly from Herrera we can only speculate as to why he scrapped the slider. One thing is for certain, he will need to use it more in 2017 if he’s to find consistent success.