Molina tweaked his back when he turned to avoid an inside pitch from Cubs closer Carlos Marmol in the ninth inning of Saturday's 5-4 win.
Cardinals manager Mike Matheny said Sunday that Molina felt better but was still a bit tight and needed a day off.
Right fielder Carlos Beltran isn't starting because of what Matheny suspects is food poisoning. Beltran hit the game-tying home run in the ninth off of Marmol on Saturday to send the game to extra innings.
Matheny says Beltran will likely be available.
St. Louis enters play Sunday leading the second wild card by 2½ games over the Milwaukee Brewers.