The Cardinals lost Geronimo Pena before Sunday's 7-2 victory over the Phillies at Jack Russell Stadium when he was injured during infield practice.
Pena, who was a steady backup at second base and stood out as a pinch-runner in his rookie year last season, broke his collarbone when he tripped over his glove while fielding grounders. He landed on his left shoulder.
"I didn't know what happened," St. Louis manager Joe Torre said. "Geronimo was finishing up the last of his grounders, and all of a sudden he went down."
Torre said Pena is expected to be out for at least two months and possibly three.
"We should have an idea (today) of how long he will be out," Torre said.
Torre was counting on Pena to be a key reserve for the Cardinals at several positions. Pena played in 83 games last season and made 39 starts at second base.
"He gave us that late-inning threat and we had the capability to use him in a lot of different situations," Torre said.