When the Yankees acquired Didi Gregorius in December 2014, one of the early concerns regarding his development as an everyday player was whether or not he could adjust to hitting left-handed pitching. During the 2014 season, Gregorius hit only .137/.228/.196 in 58 plate appearances against southpaws. However, since coming to the Bronx, he has improved markedly.
In 2015, he upped his output to a .247/.311/.315 slash line and a .626 OPS over 164 plate appearances. Over the course of just 39 plate appearances, Didi has continued to improve in 2016, evidenced by his .807 OPS against left-handed pitchers, and in the meantime, he's struggled against righties with a .542 OPS.
Much of Gregorius' improvement against lefties in the early stages of 2016 stems from his willingness to hit to the opposite field when he is pitched away. The heat map below, courtesy of Brooks Baseball, shows Gregorius' linedrive rate against left-handed pitchers per balls batted into play. Since the 2015 season, he has done well to reach out and go the other way to produce line drives on pitches to the outer half of the zone.
This perspective is confirmed by the spray chart below, which tracks Gregorius' balls put into play since the beginning of the 2015 campaign. Red marks indicate line drives, and as can be seen in the figure, a substantial proportion of Gregorius' line drives against southpaws have been directed up the middle or towards left field. This is a clear sign of maturation for Gregorius as a hitter. Earlier in his career with the Diamondbacks, he neglected to adjust his approach against southpaws. When Gregorius was pitched away, he maintained his efforts to pull the ball, leading to many softly hit ground balls to the right side of the infield. Now, by demonstrating a willingness to use the entire field against lefties, Gregorius has made himself less predictable to defend against, and is more regularly able to make the kind of hard contact that results in linedrives and hits.
Gregorius is still just 26 years old, and last season was his first as an MLB regular. He still makes mental errors at the plate, on the field, and on the base paths, but he also has yet to reach his full potential, both offensively and defensively.
One of the Yankees' most significant priorities this season has to be the continued development of Didi Gregorius. He is an integral piece of the team's youth movement, and whether that movement is successful will depend in part on the type of player that Gregorius ultimately becomes. On numerous occasions this year, Joe Girardi has opted to rest Gregorius against lefties in favor of the right-handed hitting Ronald Torreyes. Torreyes is a fine bench player, and has acquitted himself well in limited action this season, but benching Gregorius is a mistake.
Not only because he has demonstrated that he can be successful against left-handed pitching, but because the franchise is counting on his development as a complete player at shortstop. Gregorius is young, talented, and still improving, and needs to be in the lineup every time the Yankees take the field in 2016.