Complex sentence: While with the Cubs, Durocher encountered a difficult dilemma in regard to aging superstar Ernie Banks, whose injured knees made him a liability but whose legendary status made benching him impossible.
Split into simpler sentences: While with the Cubs, Durocher encountered a difficult dilemma in regard to aging superstar Ernie Banks. While Banks' bad knees made him a liability, his legendary status made benching him impossible.