He is scheduled to have surgery Friday and will miss 8-to-12 weeks.
Molina suffered the injury during the second inning of Wednesday's 5-2 win over the Pittsburgh Pirates while sliding into third base. He caught another half-inning before departing for a pinch-hitter in the third.
St. Louis recalled catcher Audry Perez from Triple-A Memphis prior to Thursday's tilt with Pittsburgh to replace him.
Molina, who was voted to his sixth All-Star Game Sunday and fourth as a starter, is hitting .287 with seven homers and 30 RBI so far this season.