Arenado (back) isn't starting Saturday's game against Philadelphia, Lynn Worthy of the St. Louis Post-Dispatch reports.
Arenado had to leave Friday's contest after his back tightened up on him mid-game, and the issue is apparently serious enough to warrant him missing a start Saturday. Nolan Gorman will start at third base in Arenado's place while Tommy Edman starts at second and Luken Baker serves as the Cards' DH.