The Cardinals placed Molina on the 15-day disabled list Wednesday and purchased the contract of catcher Brian Esposito from Triple-A Memphis. Esposito, 28, will be making his major league debut.
The team's medical staff described Molina's injury as a non-displaced fracture that will be splinted and immobilized. Molina left Tuesday's game against the Rockies in Denver in the fourth inning after being hit by a foul tip an inning earlier.
"This is a tremendous setback," Cardinals general manager Walt Jocketty said.
Manager Tony La Russa compared the loss of Molina to that of pitcher Chris Carpenter, who hasn't pitched since the season opener. Carpenter has been out since April following right elbow surgery.
"Carpenter represents more than just a starting pitcher in the rotation," La Russa said. "He is a guy that sets our tempo. Molina does the same thing behind the plate."
Molina, the youngest of three catching brothers in the major leagues, was batting .273 with one homer and 12 RBIs.
Backup catcher Gary Bennett will receive most of the work. He's batting .227 in 44 at-bats with two RBIs.
"Gary is a very good catcher," Jocketty said. "He will do a good job."
Jocketty said the team might look outside the organization for another catcher.
"I don't enough know what free agents are out there," Jocketty said. "Catching is extremely hard to find."