Complex sentence: Mathis made his major league debut with the Rangers on May 12,, when he entered the game against the Seattle Mariners in the 10th inning and was the winning pitcher in the Rangers' 13--12 victory.
Split into simpler sentences: Mathis made his major league debut with the Rangers on May 12,. Mathis entered the game against the Seattle Mariners in the 10th inning and was the winning pitcher in the Rangers' 13--12 victory.