Why did Didi Gregorius struggle at Yankee Stadium in 2015?

For Didi Gregorius, 2015 was an unmitigated success. The Yankees' newly minted shortstop struggled out of the gate, but a sterling second half propelled him to the best year of his career. By both FanGraphs and Baseball Reference WAR calculations, Gregorius ranked as the 7th most valuable shortstop in the majors. That New York may have found itself a near all-star caliber shortstop at the cost of just Shane Greene looks like a potential coup.

Gregorius improved virtually across the board. He bounced back from a .226/.290/.363 slash line in 2014, good for a 76 wRC+, to post a .265/.310/.370 line and a 89 wRC+ in 2015. His defense is what truly brought rave reviews, however, as he rated 14.2 runs above average in the field by UZR, and posted the highest defensive WAR by a Yankee shortstop since 1994, according to the Baseball Reference Play Index. Gregorius had an unenviable task in replacing a legend in Derek Jeter, but seeing the shortstop position played with such aplomb was a breath of fresh air after years of Jeter stumbling around in the middle infield.

Yet, there was an odd hole in Gregorius' game: he couldn't hit at home. This is quite a strange development for a left-handed batter at Yankee Stadium, home of the famously short porch in right field. The rest of the Yankee lineup flourished in the friendly confines of home. Alex Rodriguez, Mark Teixeira, Brett Gardner and Brian McCann all posted a wRC+ greater than 130 at the Stadium, with Carlos Beltran, Greg Bird, and Chris Young also posting above average lines.

On the other hand, Gregorius was awful at home, hitting to the tune of a 69 wRC+, better only than Stephen Drew among the Yankees' regular starters. Gregorius home line of .225/.283/.341 paled in comparison to his .306/.355/.399 line on the road, where he posted a strong 109 wRC+. Yankee Stadium has long been a haven for left-handed hitters to excel, so why, during his finest season thus far, did Gregorius struggle so mightily there?

Let's dig a little bit deeper into his home/road splits to try and find an answer. Here, compiled from FanGraphs, is Gregorius' batted ball data from 2015:

His fly ball rate at home was higher than on the road, and his ground ball and line drive rates both fell at home. To go along with the increase in fly balls, he also saw his infield fly percentage increase, along with his home run to fly ball ratio.

What to make of this? The sample sizes aren't huge, and the differences are somewhat subtle, but it seems very possible that Gregorius had a slightly different approach when playing at Yankee Stadium. Left-handed hitters have real incentive to take aim at the right-field wall that stands merely 314 feet away from home plate at its closest. Perhaps Gregorius responded to this incentive by altering his strategy when batting at home, attempting to hit more fly balls and take advantage of that tantalizing short porch.

If Gregorius actually did undertake that change in approach, it didn't work. While he did hit more home runs at home (6) compared to on the road (3), to go along with an increase in home run to fly ball ratio, the overall impact on his offensive production at home was disastrous. As he hit more fly balls, he also hit more pop-ups, and hit fewer ground balls and line drives. This torpedoed his BABIP at home, which fell from .344 on the road to .250 at Yankee Stadium.

Gregorius is not a power hitter. He owns a .115 isolated slugging mark for his career, well below league average. In order to succeed at the plate, Gregorius will likely need to run high BABIPs by peppering defenses with hard-hit line drives and ground balls, which correlate more strongly with BABIP than do the fly balls that are more likely to lead to home runs. If Gregorius changed his approach at home, focusing more on trying to send fly balls over the right field fence, that likely took him away from his most likely avenues to success as a quick, somewhat slap-hitting shortstop.

Interestingly enough, this was not a one-off phenomenon for Gregorius in his career. He also struggled mightily at home while playing at Chase Field for the Arizona Diamondbacks. On the road during his career as a Diamondback, Gregorius posted a 99 wRC+ and a .309 BABIP. At home? His wRC+ fell all the way to 70, and his BABIP to .241. Additionally, the batted ball home/road splits Gregorius has displayed as a Yankee were prevalent during his time in Arizona. Gregorius saw his fly ball percentage spike at home, while his line drive rate fell. Chase Field is similar to Yankee Stadium, in that it is both hitter-friendly and a good place for lefties to hit home runs, though not quite to the same extent as Yankee Stadium. Is it plausible that Gregorius has spent the entirety of his short career aiming for the fences at home, to the detriment of his overall offensive production?

It's impossible to say for sure. The differences in Gregorius' career output at home and on the road could be the result of simple random variation. However, if fixing Gregorius' woes at Yankee Stadium is as simple as a slight change in approach, the Yankees would be elated. Should Gregorius find a way to hit at home the way he hit on the road in 2015, the Yankees could have a star on their hands. We've already discussed Gregorius here as a breakout candidate in 2016. If his bat further catches up with his already strong glove, Gregorius has potential even beyond what he showed during his first year in New York.