Rosen played for the Pittsfield Electrics, where he was initially given a back-up role. Upon leading the Canadian–American League in home runs (16) and RBIs (86), while batting .323, however, he was bestowed his idol Hank Greenberg's nickname, the "Hebrew Hammer". Rosen played for the Oklahoma City Indians of the Texas League in , and had one of the finest individual seasons in league history. He led all hitters in average (.349), hits (186), doubles (47), extra-base hits (83), RBIs (141), total bases (330), slugging percentage (.619), and on-base percentage (.437). He won the Texas League Player of the Year Award. Rosen played for the New York Yankees' Kansas City Blues farm team in 1948. He was loaned to the Blues for the duration of the team's American Association season as part of a deal that sent reliever Charley Wensloff from the Yankees to the Indians. Rosen was named Rookie of the Year in the American Association for his play with the Blues. Major League Baseball career Rosen made his first appearance in the major leagues in at the age of 23. In 1948, Rosen played most of the year in minor leagues with the Kansas City Blues, before joining the Indians in September. He played in the 1948 World Series as a reserve behind regular third baseman Ken Keltner despite only playing five games during the season after the Indians requested his inclusion on the World Series roster. When Keltner was traded in , Rosen took over as the Indians' third baseman, leading the American League in home runs with 37, hitting more than any previous American League rookie. It stood as the AL rookie record until Mark McGwire surpassed it in 1987. He homered in four straight games in June, a feat unmatched by an Indians rookie until Jason Kipnis in 2011. Rosen averaged a league-best homer every 15.0 at bats, and led the league as well in HBP (10). He batted .287 and had 116 runs batted in, while finishing fifth in the league with 100 walks and a .543 slugging percentage. His 100 walks remained a team rookie record for a right-handed batter, through 2014. He also remained the most recent AL rookie to record at least 100 walks until Aaron Judge in 2017. Despite his home run title, he finished 17th in the American League MVP Award voting. In Rosen led the league in games