Complex sentence: During the 1940 season, he was taught how to throw a slider by teammate Johnny Allen, but he Milnar injured his pitching shoulder throwing it in 1941 and his career suffered as a result.
Split into simpler sentences: During the 1940 season, Milnar was taught how to throw a slider by teammate Johnny Allen. However, Milnar injured his shoulder throwing the slider during the 1941 season, and his career suffered as a result.