BOSTON — The idea was to double down on a skill set. The Yankees already had Brett Gardner — speed, defense, on-base skills — when they signed Jacoby Ellsbury to a seven-year, 3 million free-agent deal following the 2013 campaign. In the best-case design, the Yankees envisioned a distracting 1-2 atop the lineup in front of their RBI muscle and two fleet outfielders covering huge terrain to rob extra-base hits. It hasn’t worked as imagined. Ellsbury has been hurt a lot and never could sustain elite play. Plus, the sport went through a launch-angle transition that made it more difficult to simultaneously play two outfielders who lacked power. The Yankees doubled down on a skill set again this