Sizemore was making his second start since the 2011 season when he suffered the injury Tuesday in the team's 9-5 victory over the Los Angeles Angels. Sizemore had one hit in six at-bats in two games with the A's before the injury.
Surgery has not been scheduled for the 28-year-old, and it is not known when he will return to action. Sizemore missed the entire 2012 season after tearing the ACL in his left knee during spring training.
The team put Sizemore on the 15-day disabled list yesterday not knowing the extent of his injury.
Eric Sogard will serve as the A's primary second baseman for the time being. The team also called up Andy Parrino from Triple-A Sacramento. Sizemore spent two seasons with the Detroit Tigers and is a career. 238 hitter in 518 at-bats.