Didi Gregorius is on his way back to the Yankees. His rehab from the shoulder injury he suffered while playing for the Netherlands in the World Baseball Classic is nearly complete, and he is due to join the team today. Ronald Torreyes has performed admirably in his stead, but given how tough it is to ask a reserve like Torreyes to continue to play this well, Gregorius' return doesn't come a moment too soon.
He returns on the heels of perhaps his best season as a pro. His .276/.304/.447 slash line equated to a 97 OPS+, the highest such mark of his career. His breakout was clearly propelled by a surge in power, as Gregorius joined the legions of players sending more and more balls over the fence. He set a career-high with 20 homers, eleven more than his previous career-best.
League-average offense and quality defense at shortstop made Gregorius one of the Yankees' best players. The question going forward, of course, is whether Gregorius can prove his progress at the plate was no fluke, and continue to hit at what amounts to a strong level for a shortstop.
Which naturally leads us to the question of whether Gregorius can sustain his power surge. Part of Gregorius' 20-homer outburst can be explained by league-wide power trends, but not all of it. Surely, Gregorius must have done something to post a .447 slugging percentage despite entering the year with a career slugging figure below .370.
One way to hit for more power is to simply hit the ball harder. Home runs are almost exclusively well-struck batted balls, so hitting well-struck balls more often should lead to more home runs. However, Gregorius' average exit velocity of 85.7 mph last year was unimpressive, as was his average exit velocity of 88.6 mph on fly balls and line drives. His overall hard contact rate, per FanGraphs, was also below average at 24%, right around his career norms.
So Gregorius wasn't hitting the ball with authority to hit more home runs. Perhaps he simply hit more balls in the air? That hypothesis doesn't pass the smell test either. His fly ball rate did jump from 34% during 2015 to 40% last season, but his fly ball rate the previous two years with the Diamondbacks was greater than 42%. He managed just 13 total homers across those two years, so it doesn't seem like we can chalk up his increased power output to either hitting the ball harder, or hitting the ball in the air more.
Maybe Gregorius molded his lefty swing to take advantage of the short porch in right field at Yankee Stadium. Again, the evidence is unconvincing. He pulled the ball slightly more at home (39% pull rate compared to 37% overall), and had a slightly higher fly ball rate at home (41% compared to 40% overall), but both marks are close to his averages, and probably not enough to indicate a tactical change. Indeed, Gregorius hit just two more home runs at home than on the road, a very minor disparity.
There really doesn't seem to be much to indicate that Gregorius changed in a meaningful way in order to produce more power. This is all to say that unless Gregorius does change something this year, he probably won't be as effective a hitter, and he probably won't reach his ceiling of a solid offense/great defense shortstop.
Which is not to say he isn't capable of making a change! As we just highlighted, there were a number of reasonable ways Gregorius could have pushed himself to newfound levels of power last season. 2017 could be the year that Gregorius hits 20 home runs because he starts hitting the ball harder, or because he starts tailoring his swing to hit more fly balls into those inviting right field bleachers at Yankee Stadium.
Or it could be the year he learns how to take a walk. Gregorius was hyper-aggressive at the plate last season, posting a career-high swing rate of 55%. This wasn't a case of swinging a lot at pitches in the zone and avoiding balls: his out-of-zone swing rate was among the ten highest in the league, contributing to his minuscule walk rate. If Gregorius' power regresses this year, one way he could counteract that would be to cut back on his aggression at the plate and try to bump his OBP up.
There are plenty of routes Gregorius could take to being a fine hitter, as he was last season. It just doesn't seem like his best route to notable offensive production is the one he used in 2016. Gregorius probably needs to change in order to stay at the level he set last year. If Gregorius doesn't hit the ball hard and doesn't learn how to let a bad pitch go by in 2017, he's likely to take a small step backward, even if he remains a valuable player.