Complex sentence: During April of that season, he allowed just one earned run in 12 ⅔ innings pitched; despite that, his performance declined and he finished the season 3--5 with a 5.90 ERA in 67 appearances.
Split into simpler sentences: During April of that season, he allowed just one earned run in 12 ⅔ innings pitched. But his performance declined and he finished the season 3--5 with a 5.90 ERA in 67 appearances.