Gio Gonzalez is replacing strikeouts with grounders — but why?

Baseball players change a lot. Everyone knows this; only an idiot would begin a piece by reiterating it. The reasons behind these modifications, though, aren't always clear, and baseball fans (especially the analytic-minded ones) can often waste spend a lot of time pondering them.

For Gio Gonzalez, the 2015 season has brought mixed results. On the one hand, he presently owns the highest qualified ERA- of his career, at 123. On the other hand, his FIP- ranks 11th in the NL and his xFIP- 10th. We'll discuss the former facet of his current campaign in a moment, but for now we'll focus on the latter. Since 2010, when Gonzalez first qualified for the ERA title, his xFIP- has stayed in the mid-90s, with a flukish dip slightly below 80 in 2012. This metric has gauged his skill as that of an above-average, #3-ish starter. How has he made the leap from there to possible #1?

First, let's look at Gonzalez over that first five-year span. In that time, he struck out 23.2 percent of the batters he faced, in the 89th percentile among fellow qualifiers. He also accumulated ground balls on 46.9 percent of the balls in play against him, good enough for the 62nd percentile. So he fanned a superb amount of batters and netted a decent rate of grounders to go with them.

That recipe has disappeared in 2015 in favor of a wholly different one. Across his 59.0 innings thus far, Gonzalez has a (comparatively) mediocre 21.9 percent K rate, which translates to the 56th percentile. However, he's made up for that with a 56.3 percent ground ball rate, giving him a 94th percentile rank. In other words, he now receives grounders from a superb amount of batters and nets a decent rate of strikeouts to go with them.

The causes for this change don't take much digging to find. For the first five full years of Gonzalez's career, the four-seam fastball served as his primary pitch with the sinker there to supplement it. Year six has seen that trend flip:

Suddenly, Gonzalez is a sinkerballer who backs the eponymous pitch up with a regular heater. Because batters whiff at four seamers far more often than they do at sinkers, the absence of the former takes some strikeouts away from Gonzalez, for which he compensates with the additional grounders netted by the sinker.

Pitch selection doesn't account for the entirety of Gonzalez's alteration, though. His location has also shifted downward, even on his non-dropping pitches:

Low pitches result in fewer swinging strikes and also tend to stay on the ground when put in play. This, along with the repertoire adjustment, has effected Gonzalez's transformation.

By his peripherals, Gonzalez has never pitched better than he has to start 2015. But the question remains: Why would he revise himself in this manner? He certainly didn't struggle beforehand, and more importantly, he consistently demonstrated the capacity to outperform his estimators. The aforementioned 92 xFIP-? That accompanied an 82 ERA-, which graded him as a marquee starter. Compare that to his current 123 ERA-, which borders on replacement level.

Gonzalez hasn't yet pitched 60 innings in 2015, and he compiled more than 900 in the preceding five seasons, so the sample size matters here. As the year progresses, his .379 BABIP will come down to earth, and so will his ERA. Nevertheless, we shouldn't expect him to prevent scoring like he once did. Heavy ground ball pitchers tend to underperform their peripheral statistics; as such, Gonzalez will probably meet a similar fate. Plus, his teammates won't help his transition: Washington's infielders have a combined -3.3 UZR and -8 DRS in the present season. (For what it's worth, the outfielders have fielded poorly as well, with -1.3 UZR and -7 DRS in their 1320.0 innings.) With this team behind him, and this profile guiding him, Gonzalez will be lucky to post an above-average ERA — and that's assuming the ground balls don't regress away.

Like most baseball players, Gonzalez has evolved for the new season. Unlike most players, Gonzalez doesn't seem to have much justification for this change, and the coming months presumably won't help his case. Ground balls for strikeouts can sometimes be a worthwhile exchange, but in this case, it probably will end up a regrettable one.

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All data current as of June 1st.

Ryan Romano is an editor for Beyond the Box Score. He also writes about the Orioles on Camden Depot and on Camden Chat that one time. Follow him on Twitter at @triple_r_ if you enjoy angry tweets about Maryland sports.