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Mike Fiers’s overnight transformation

How an altered arm slot and repertoire have netted big returns for the veteran Astros starter.

Minnesota Twins v Houston Astros Photo by Bob Levey/Getty Images

No one would blame you if you thought Mike Fiers was still Mike Fiers: an adequate yet decidedly unexceptional starting pitcher for the Houston Astros.

No one would blame you if you thought he was bad, even, after he opened the season with baseball’s worst home run rate (3.5 per nine innings) and second-worst FIP (7.69) through May 25.

Certainly no one blamed the Astros when they sent Fiers to the bullpen after that. But just hours into the demotion, Charlie Morton was forced to the disabled list with a lat injury, and Fiers was placed back into the rotation.

That’s when everything changed. According to the Houston Chronicle’s Jake Kaplan, Fiers decided to lower his arm slot (which would allow him to throw his curveball harder), ditch his cutter (which had been getting hit hard), and throw more changeups.

Since that time, he has a pristine 2.36 ERA in 61 innings, the legitimacy of which is more or less backed up by a 2.80 FIP and 3.60 xFIP.

Here’s how Fiers’ pitch usage has changed since May 30:

Mike Fiers Pitch Usage

Split IP FB% SL% CT% CB% CH%
Split IP FB% SL% CT% CB% CH%
4-7-17 to 5-25-17 46.2 39.5% 10.5% 16.4% 16.9% 16.7%
5-30-17 to Present 61.0 46.6% 0.7% 8.5% 21.0% 23.3%
Courtesy of FanGraphs

Noticeable differences everywhere. He’s throwing his slider and cutter less, and featuring his fastball, curveball, and changeup more. The results have been impressive:

Mike Fiers Results

Split IP K% GB% HR/9 HR/FB xwOBA
Split IP K% GB% HR/9 HR/FB xwOBA
Career before 5-25-17 619.1 22.1% 38.0% 1.40 13.7% Unknown
4-7-17 to 5-25-17 46.2 18.4% 43.9% 3.47 31.0% .368
5-30-17 to Present 61.0 27.9% 46.0% 0.44 6.1% .294
Courtesy of FanGraphs and Baseball Savant

There are wholesale improvements, the most significant of which is the amelioration of what had become a nasty home run problem. Fiers’s uptick in ground ball rate offers the best explanation, although his ground ball rate was up this year even when he struggled. No matter how he’s done it, Fiers has surrendered just three home runs in his last 10 starts after allowing 18 in his first nine.

The Statcast data suggest that Fiers’s improved batted ball profile has little to do with luck. Since May 30, the league’s expected wOBA against Fiers has dipped 76 points. (Expected wOBA is calculated using launch angles and exit velocities to predict wOBA.)

Not only has Fiers been demonstrably better at inducing weak contact, getting ground balls and allowing fewer home runs, he’s also become a legitimate strikeout threat. His 27.9 percent strikeout rate since May 30 is sixth-best among American League starters in that span and puts him on par with James Paxton, Luis Severino, and Yu Darvish.

The improvements look legitimate from multiple angles. There’s a chance that the league will adjust to the new Mike Fiers and he’ll be forced to go back to the drawing board, but for now, he’s become an exciting and interesting player to watch.

In the Astros’ rotation, Charlie Morton has returned from the disabled list and Dallas Keuchel is expected to be back later this week. Keuchel’s return complicates things for Fiers, who could be forced back to the bullpen despite his resurgence.

Whatever happens, if the Astros’ biggest problem with Mike Fiers now is that they may have to move an exciting and interesting arm to the bullpen, they’re in an enviable position indeed. It’s a position that probably seemed unimaginable mere months ago, when Mike Fiers was still the old Mike Fiers.

All stats current through July 24.

Ben Kaspick is a contributor for Beyond the Box Score and RotoGraphs, and the owner-operator of CoveCast, a saber-slanted San Francisco Giants podcast. You can follow him on Twitter @benkaspick or @Cove_Cast.