They could also be jiggering the rotation for the playoffs and they only have one off day left.
If he pitched today, he'd pitch the final game of the Braves series, the first game of the Cardinals series and his final start of the regular season would be the second game of the Diamondbacks series. He couldn't possibly start in the play-in game.
EDIT - His last start would be the final game of the season against the Dbacks. Starting him tomorrow would line him up to start the play-in game.
