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.
I like the way you're thinking. Don't waste SS on the Phillies. We can/will beat them.