Rosar was hitting for just a .216 batting average by mid-season in 1948, but his defensive reputation won him the fans' vote as the American League's starting catcher in the 1948 All-Star Game. During a three-season period between 1946 and 1948, Rosar committed only three errors. 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.