 By 1949, Mike Guerra had taken over as the Athletics starting catcher and, Rosar would be traded to the Boston Red Sox in October 1949.With the Red Sox, he was the third string catcher behind Birdie Tebbetts and Matt Batts in 1950 and then to Les Moss in 1951 before being released in October 1951.In a thirteen-year major league career, Rosar played in 988 games, with 836 hits for a .261 career batting average, along with 18 home runs and 367 runs batted in.