Complex sentence: He was named the defensive player of the month for December by the league, but suffered a sprained medial collateral ligament in his knee at the end of the month that forced him out of the lineup for four weeks.
Split into simpler sentences: He was named the defensive player of the month for December by the league. That stretch ended when he suffered a sprained medial collateral ligament in his knee that forced him out of the lineup for four weeks.