The New York Yankees entered the offseason with multiple holes on the roster, but none more gaping than shortstop. There were no credible internal options to be the regular starter in 2015, Brian Cashman had to look externally and with few appealing free agent options available, he worked a three-team trade to obtain Didi Gregorius.
After making the trade though, Cashman suggested that the Yankees might look to platoon newly-acquired shortstop Gregorius with a player already on the roster, Brendan Ryan. Looking at Didi's career splits against right and left-handed pitching, it starts to become clear why the idea of a platoon might make sense.
First, his performance against right-handed pitching in 544 plate appearances which effectively scales to one full season.
0.60 BB/K, 13 HR, 0.149 ISO, 0.743 OPS, 0.324 wOBA, 102 wRC+
That line indicates a league-average hitter against two-thirds of starting pitchers. This coupled with the ability to provide solid defense at shortstop is incredibly valuable. Giving up five seasons of a potential quality starting pitcher in Shane Greene to acquire Gregorius was a steep price, but this shows why Brian Cashman chose to pay it.
Note: wRC+ is a scaled attempt to measure a player's participation in creating runs, 100 is league-average and every point in either direction is a shift by one percentage point from the average.
As a left-hander himself, Gregorius would be expected to show some drop-off from the numbers above when facing fellow southpaws. However, perhaps not quite this much...
0.29 BB/K, 0 HR (!), 0.049 ISO (!!), 0.490 OPS, 0.226 wOBA, 33 wRC+
First, a caveat, the above is over only 180 plate appearances. It is worth noting that his batted average on balls in play (BABIP) against lefties is a not horrible 0.254. For reference, his BABIP against right-handers is 0.287, certainly higher but well within the expected range considering what that difference in isolated power suggests about the type of contact he is making against left-handers. This certainly doesn't excuse away the small sample size alert, but does suggest that better luck won't significantly improve his numbers against lefties. Not unless it also comes with some power and a reduction in strikeouts.
Of course, Gregorius is still a young player and can improve, and once again 180 plate appearances is too small a sample size from which to draw meaningful conclusions about a player's career. However, certainly one can see the appeal in sitting Gregorius against left-handed pitching, at least to begin the 2015 season. Find Gregorius a right-handed platoon partner who can play shortstop adequately while providing close to average offense against left-handed pitching, and the Yankees will have turned shortstop from a position of weakness into one of the strongest areas on the depth chart.
There is of course, a problem with this. The Yankees don't have such a player. This is hardly a surprise; it's rare to find league-average level hitting shortstops even from one side of the plate who aren't already holding down a full-time job. Ryan is the only other shortstop on the roster which makes him the incumbent. Although an incumbent right-hander who made only 25 plate appearances against left-handed pitching last year, admittedly for better or worse a certain Derek Jeter was going to play against as many left-handers as possible. Ryan posted reverse-splits in 2014, struggling against lefties to the tune of negative (!!!) 36 wRC+ but of course 25 plate appearances barely counts as a sample size. Still. Brendan Ryan hasn't been even an average hitter against left-handed pitching since 2011. Or over his career. He has posted a 75 wRC+ after 944 plate appearances against lefties and he's been 25% worse than league average at hitting left-handed pitching. Even that number is inflated by a very impressive 2007 (132 wRC+, 113 PA) far too long ago to have any bearing on 2015.
Brendan Ryan isn't really the answer, not if he's asked to do more than occasionally fill in. It might simply be better to just let Didi take his lumps against lefties and see what he can become. Perhaps use Ryan against the toughest lefties, give Gregorius some rest and protect his confidence until he starts to show better performance against left-handed pitching overall, but no more than that.
However, should the Yankees acquire an infielder who can play shortstop and actually hit left-handed pitching, then perhaps this changes the equation. Cashman is unlikely to part with valuable assets to acquire a platoon player who will play around 40 games at most, especially after giving up Shane Greene to bring in one young, cost-controlled shortstop. Free agents would likely choose to go somewhere where they have a better chance to earn playing time. Having just traded for Gregorius the Yankees are likely invested in him as the shortstop for the next five seasons, or at least that should be the plan. Additionally, adding another shortstop to the roster will likely mean the Yankees will have to waive Ryan and swallow the $2.5 million he is owed for next season.
There might be a possibility of acquiring an infielder who might be able to play primarily second or third base, with the ability to cover shortstop as well. This might have been the motivation behind the rumored interest in Dee Gordon for example. At this point though, this seems unlikely. Of course Cashman is always a threat to make the unexpected move. Barring said unexpected move, expect to see the Yankee shortstop facing left-handed pitching in 2015, be it Ryan or Gregorius, to be making a lot of outs.