At the age of 23, he made his first major league appearance for the Oakland Athletics in September 1987. The club was impressed enough with the young shortstop's talent that they traded starter Alfredo Griffin that December, making him their starting shortstop for 1988. His offensive numbers were low (.250 average, three home runs, 39 RBIs and 44 runs scored), but his defensive wizardry helped lead the A's to their first American League pennant since 1974. The 1988 World Series was a rematch of the 1974 matchup, with the Los Angeles Dodgers winning the National League pennant. His costly error in Game 4 helped the Dodgers win the Series in five games, but he was voted American League Rookie of the Year for 1988 as the third consecutive Oakland player to win the award after sluggers José Canseco in 1986 and Mark McGwire in 1987. He also made the 1988 Topps All-Star Rookie Roster.