If the Yankees make it to Game 5 in the ALDS against the Red Sox on Thursday in Boston, Aaron Boone wasn’t clear heading into Game 4 on Tuesday night at Yankee Stadium about who would start the decisive game.

J.A. Happ, who started Game 1 and lost, and Masahiro Tanaka, who started Game 2 and won, were available in relief for Game 4, and if either or both pitched, that would shape who would start Game 5.

“Happ would be available and we probably would have Masa on some level available, too,’’ Boone said of Game 4. “Happ would be a guy, especially if we got through the game in extra innings. Happ could give us significant length. Masa would be more emergency.’’

Obviously, if the Yankees won Game 4 and Happ went multiple innings two days before a start, Tanaka would likely get the Game 5 assignment on four days rest.

However, if neither hurler was needed in Game 4 and the Yankees won, Boone was undecided who would start Game 5.

“If we don’t use Happ or Masa [in Game 4], we haven’t decided [Game 5 starter] yet,’’ Boone said.