Rosar was first discovered in 1934 when he was chosen to play in an All-Star game for amateur baseball players from Buffalo, New York. The wife of New York Yankees manager, Joe McCarthy, attended the game and was so impressed with Rosar's catching ability that she told her husband about him. McCarthy sent Yankees' scout, Gene McCann to look at Rosar before the team signed him as an amateur free agent. He played for the 1937 Newark Bears team that won the International League pennant by 25½ games to become known as one of the best minor league teams of all time. Rosar hit .387 with the Bears in 1938 to win the International League batting championship.