Complex sentence: In his first thirteen games with the Rangers, they had a 1--12 won-loss record, while Nevin batted 5-for-44, which lowered his season batting average under.200, known as the "Mendoza Line".
Split into simpler sentences: In his first thirteen games with the Rangers, the team posted a 1--12 record as Nevin batted 5-for-44. That put his season batting average under.200, known as the "Mendoza Line".