Mike Fiers has a home run problem

It’s been a difficult season for Mike Fiers.

Through six starts and just 30 ¹⁄₃ innings, Fiers has allowed 14 home runs. That puts him tied for the the league lead with the soft-tossing Jered Weaver — inauspicious company to be sure. While tied with Weaver for the raw total of long balls allowed, Fiers stands alone as the owner of baseball’s worst home-run-to-fly-ball rate. For some context, here’s the bottom 10 pitchers in this statistic who’ve thrown at least 30 innings.

As you can see, Fiers doesn’t just have the league’s worst home run to fly ball rate, he has the worst BY FAR. When an opposing hitter has hit a fly ball against Fiers this season, it has left the yard a staggering 41.2 percent of the time. From 2014 to 2016 he saw an increase in this rate each year — from 8.4 to 11.3 to 15.2 percent — but his current number is absurd.

What’s been different this year? His overall fly ball rate is up from 2016, but still well below his 2014 and 2015 marks. There hasn’t been a significant increase in his hard hit rate allowed, but batters are pulling the ball against Fiers more than ever.

That squares with what’s been happening, as most hitters exhibit more power to their pull side. A change in a batted-ball profile like that probably doesn’t just happen on its own. It stands to reason that something has changed with the way Fiers is attacking hitters this season. Looking at his pitch type usage shows a significant change.

Fiers is throwing his four-seam fastball less than ever, while increasing usage of both his slider and cutter. The problem is that the two pitches just aren’t that different. His cutter is generally about three to five miles per hour faster than his slider with anywhere from three to five more inches of rise. But recently, they’ve begun to bleed into each other with regard to horizontal movement. The slider and cutter have basically had identical run for his past three starts.

So Fiers has changed his pitch type usage significantly, but the two pitches he’s relied on more than ever aren’t that different. This shift in approach has led to a couple of small but telling changes in opposing hitters’ plate discipline numbers.

Batters are swinging at basically the same rate against Fiers, but they are making less contact overall. The problem is the decline in contact is coming almost exclusively on pitches outside of the zone. From 2016 to 2017, contact against him in the zone has decreased by just .2 percentage points, compared to 10.7 percentage points outside. Put simply, Fiers has struggled to get batters to chase like he has in the past.

So there’s been a change in how Fiers is attacking hitters and how hitters are responding. Still, a 41.2 home-run-to-fly-ball rate is insane. Some of those dingers had to be flukes, right? Let’s see.

Based on expected home run percentage from Statcast — using the combination exit velocity and launch angle — only two of the long balls against Fiers leave the yard less than 20 percent of the time, and only two are gone more than 80 percent of the time. Overall, Fiers has an average expected home run percentage of 53 percent, which anecdotally seems like what we should expect from a normal home run distribution. At the end of last season I calculated average expected home run percentage for hitters and the league average mark was 59.1 percent.

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Perhaps what’s most crazy about Fiers’ season so far is that despite the home runs, he’s actually getting lucky and outperforming his peripherals. Sure his 5.64 ERA is ugly, but it’s far preferable to his 8.57 FIP or 8.39 DRA. Fiers is sporting a career-high ground ball rate of 50 percent, a suppressed BABIP of .244, and a league leading left-on-base percentage of — brace yourself — 98.5 percent. For the most part, hitters are either jogging around the bases at a leisurely pace or left waiting for a teammate to bring their glove and hat for the next half inning.

The good news for Fiers is that even in the new, homer-happy environment of 2017, his home-run-to-fly-ball rate is almost assuredly unsustainable. The league average rate is currently 12.8 percent. The bad news is that both his BABIP and left-on-base percentage numbers are also likely to regress.

Even if the dingers slow down, Mike Fiers is still in trouble.