Through the first two weeks of the 2016 season, the Yankees have faced two starting left-handed pitchers, and Joe Girardi has started Aaron Hicks in favor of Brett Gardner in both of those contests. One of the primary reasons the Yankees acquired Hicks was his ability to hit left-handed pitching, and he figures to play regularly when the team faces southpaw starters.
It's difficult to assess Girardi's decision to rest Gardner both times. The sample size is extremely small, and Gardner may ultimately find himself getting plenty of chances to start against lefties as the season progresses. Still, it is difficult not to read into Girardi's decision to start Jacoby Ellsbury rather than Gardner, who like Gardner also hits left-handed.
In 2015, Gardner performed markedly better against left-handed pitching than Ellsbury. Gardner's OPS against lefties was more than 100 points higher than Ellsbury's (.761 versus .652) last season, although the two have performed comparably against them throughout their careers (.720 career OPS for Gardner and a .735 career OPS for Ellsbury). Both players are also capable outfield defenders, making defense a less helpful metric for equivocating between the two players.
One area though where Ellsbury clearly outshines Gardner is on the basepaths, particularly with regard to stealing bases against lefthanded pitching. Gardner has stolen 203 bases in his career, but just over 15% (31) of those steals have come against left-handed pitching. In 2015, Gardner stole 20 bases, with none of those steals coming against lefthanders. Gardner has not had more than four steals against left-handed pitching in a season since 2011 when he stole nine. However, in that same season, he stole 40 bases against righthanders, such that fewer than 20% of his total steals came against left-handed pitchers.
Throughout his career, as well as more recently, Ellsbury has proven a much more willing and effective base stealer against lefties. For his career, 26% (79) of Ellsbury's 304 steals have come against left-handers. In 2015, Ellsbury stole 21 bases, and seven (33%) of those steals were against lefties. During his first season with the Yankees in 2014, Ellsbury stole 39 bases total, with 10 steals (26%) against southpaws. Simply put, against left-handed pitching, Ellsbury poses a much bigger threat to steal than Gardner.
That is not to say that Ellsbury's superiority stealing bases against lefties should or is dictating Girardi's decision to start him against left-handers early in the season. Gardner was clearly the better hitter against lefties last season, and may ultimately prove to be again this year. The Yankees' best defensive outfield includes both Ellsbury and Gardner, as well as Hicks, with Carlos Beltran either sitting on the bench or serving as the designated hitter.
However, Gardner wore down in the second half of 2015, hitting for an OPS of just .592. He played in 151 games last season, and is unlikely to be allowed to reach that mark this season in an effort to keep him fresh into October. Gardner derives significant value from his base running ability, and that value is diminished when he faces left-handed pitching. As a result, Girardi's decision thus far to rest Gardner against lefties may simply reflect an effort to keep Gardner fresh, and to do so in a way that puts him in the best possible position to succeed in all aspects of his play.