Oakland Athletics third baseman Scott Sizemore tore a knee ligament and will miss the entire 2012 season, the team said Monday. Sizemore, 27, was being counted on by the A\'s to be their starting third baseman this season after batting .245 with 11 home runs and 56 RBI over 110 games in 2011. Instead, he will be put on injured reserve after tearing the anterior cruciate ligament in his left knee Saturday during Oakland\'s first full-squad workout. Sizemore was obtained from the Detroit Tigers in May for pitcher David Purcey. The former fifth-round draft pick has put up a .239 batting average with 14 homers and 70 RBI in two major-league seasons.