I've been as critical of Joe Girardi as the next guy, about his affinity for the sacrifice bunt, his love of The Binder, or his use of personal catchers for A.J. Burnett, but to be fair, these things aren't entirely indefensible; the Yankees only rank 7th in the AL in sac bunts this season, relievers do tend to pitch well in short, defined roles, and hey, you can't blame a guy for doing something he thinks might coax a visit from Good A.J.
However, the one thing I can't defend in any theoretical terms is what he's done with Brett Gardner. Gardner is the Yankees' best baserunner, their best defensive outfielder, and has been one of the most valuable left-fielders in all of baseball since 2010.
And Girardi platoons him. Regularly.
Platoon splits are a funny thing, under-acknowledged and under-used. Well-known hitters like Ryan Howard and pre-2011 Curtis Granderson have long and mostly awful track records against same-handed pitchers, yet their managers tend to ignore it regularly. Since we're picking on Girardi, remember that Granderson was rarely benched against left-handers in 2010 despite the fact that his career OPS+ and wRC+ were both less than 70 against them up to that point in his career.
As for Brett Gardner, if you put aside 2008, his rookie season in which he went 3 for 26 against lefties, here are his seasonal platoon splits:
Looking at these numbers, can anybody give me a good reason why Gardner is averaging 4.03 plate appearances this season in games started by right-handers, but just 1.8 PAs in those started by lefties? Or how about why he's in the starting lineup against good lefties like Jon Lester (career 129 ERA+), but is benched against mediocre ones like Jo-Jo Reyes (career 69 ERA+), Chris Capuano (99 ERA+), and Derek Holland (86 ERA+)?
Maybe the better question to ask is what's going on inside of Joe Girardi's head, because this goes far beyond giving somebody an occasional day off. It's still early, but the AL East is as competitive as everybody expected it to be, and with with an aging core and a starting rotation that seems to be staying afloat with smoke and mirrors, the Yankees need to be pushing every single edge they can get. It would be a shame if Girardi continues to bench one of his best players for no particularly good reason, but an absolute travesty if the Yankees miss the playoffs by a game or two while he does it.