Mike Fiers completed his Oakland redemption tour last week. Way back in July of last year, Fiers started a dreadful game against the A’s that ended in 7-4 Astros loss. His outing wasn’t pretty: Fiers didn’t get out of the fourth inning, allowed six runs and eight baserunners. He capped the outing with a minor tantrum on the mound when A.J. Hinch walked to the mound to remove him and continued this in the dugout. It was not a good look, and a ton of people called for him to be demoted or cut on that show of insubordination alone.

Fast-forward to last Tuesday back at the Coliseum - Fiers worked into the seventh, struck out five, kept the ball in the yard and finished with a win. And no tantrums. He’s been the Astros most dependable pitcher since the end of May, winning four games, lowering his ERA to 3.81 and most importantly, working deep into games - he’s averaged 6.2 innings in those five starts. Mike Fiers’ redemption mission may be complete.

For an Astros team that’s really just looking for average pitching depth with 80% of the rotation on Weiland Island at one point in June, just eating innings is valuable. But Fiers has actually been impressive since that May 30 start in Minnesota. He’s changed his arsenal a bit, refined his pitch location with that new pitch and is looking more like the dependable mid-rotation guy Luhnow traded for in 2015 instead of the homer-prone flyball pitcher he was earlier this year.

Mike Fiers’ fastball was dreadful in 2016 and earlier this year. Hitters slugged .660 slugging against his fastball in ‘16, which nearly what Aaron Judge is doing right now. Fangraphs marked his four-seam at nearly -17 runs (wFB), a linear weight that gives a run value based on how a pitcher performed with a certain pitch. This mostly continued through the early part of this season - Fiers was giving up an insane number of homers - 18 in his first nine starts (11 of those on his fastball), which made him look closer to 2000 Jose Lima than August 21, 2015 Fiers. His fastball was a little better based on Fangraphs’ wFB metric early this year (-3.8 wFB), but his other pitches were getting hit hard as well so there wasn’t much to be confident about overall. Fiers wasn’t getting unlucky just because he threw some bad fastballs - he was just bad at everything.

Then, starting in mid-May, something changed. Fiers started throwing a sinker at higher rates, and the results have shown to be neither a coincidence or insignificant.

Fiers hasn’t allowed a home run in almost 33 innings dating back to May 25. He was certainly due for some positive regression from an astronomical HR/FB rate, but it looks like a lot of this can be attributed to throwing the sinker more, and where he’s located the sinker (more on that below). Fangraphs has liked Fiers’ fastball and the results that have followed so much, he has a 7.3 wFB in his last five starts.

Fiers’ has not only changed his arsenal significantly by adding a sinker (not an uncommon occurrence with the Astros staff, or even across the majors - check this Fangraphs article on two-seamers). A pitcher can differentiate their pitches as much as possible, but if they’re not effectively located, they’ll still get hit. The below heatmap is Fiers’ four-seamer in 2017, with notably bad location - that’s a ton of 89-90 mph fastballs right over the plate.

Fiers’ made one pitch on April 27 against Cleveland that’s great example of this poor fastball location - if you watch closely in the highlight below, you can see that Evan Gattis tries to set up a first-pitch fastball to Abraham Almonte a little off the plate.

Fiers missed his location in by about two feet and the result was disastrous, but pretty predictable of an 88 mph fastball. Even a guy with a career 83 wRC+ is going to destroy that pitch. But along with adding the sinker, Fiers has changed his location with the pitch - he’s locating it near mostly near the bottom of the zone (and interestingly, up in the zone as well).

Concentrated density in the bottom of the zone with the sinker is obviously good and has resulted in an increased groundball rate for Fiers - less hard contact, fewer flyballs, stuff like that. All that is great for any pitcher - but for a guy with barely average fastball velocity and just above-average strikeout rates like Fiers, great location is paramount.

Which leads in to the concerning bit of that second heatmap on Fiers’ sinker location. See the increased density on the upper edge of the zone? Sinkers aren’t really supposed to go there - drilling down in the zone to miss bats and create weaker contact is really the overall point a two-seam/sinker offering. Location differentiation can be a good thing, but Fiers’ historical batted ball profile (flyball-heavy which leads to a ton of homers) suggests he’s playing with... *puts sunglasses on* fire when he elevates the two-seamer, so something to keep an eye on.

There’s a lot more that could go here about Fiers’ recent turnaround. He chucked his slider which was also getting hit a ton and not generating a ton of whiffs in the first place for his curveball, which he’s had tons of success with in the past month. Brent Strom also deserves a ton of credit - he’s recognized as a top pitching coach in the game, but what he’s done with a patchwork rotation of guys who were about to be demoted or released two months ago and a bunch of rookies has been savant-level stuff.

There’s also regression, where Fiers has experienced both extremes in 2017. His HR/FB rate was bound to come down, and it has - but zero homers allowed in over thirty innings is too good to be true for most pitchers, and he’s still carrying a FIP over 5.00. We’re also working with very small sample sizes, as half of a season isn't even over yet - the first quarter was very bad for Fiers, and the second quarter has been very good, and ultimately he’s still probably somewhere in between these wildly different sets of results. But even if this isn’t quite the real Mike Fiers going forward, he’s been a huge boost to a rotation in need of solid innings and a small-sample poster boy for the sinker revolution across the league.