Complex sentence: Higgins managed the Red Sox through the mideason of 1959, when he was replaced by Billy Jurges and became a special assistant to owner Tom Yawkey, with whom he had become friends.
Split into simpler sentences: Higgins managed the Red Sox through the mideason of 1959, when he was replaced by Billy Jurges. He then became a special assistant to owner Tom Yawkey, with whom he had become friends.