Who was the only major-leaguer to retire after winning 27 games the year before?

The pitcher who retired immediately after winning 27 games was Sandy Koufax. In 1966, his final season, the lefty went 27-9 with a 1.73 ERA and 317 strikeouts while winning the NL Cy Young Award, and Triple Crown for pitchers. He also led the league in innings pitched, complete games, and shutouts. However, because of arm problems, he was forced to retire the next season, at the age of 31. Because of his early retirement, Koufax was the youngest player ever inducted into the Hall of Fame at the age of 36.