 Rivera was drafted by the Seattle Mariners in the second round (49th overall) of the 2001 Major League Baseball Draft as a 17-year-old.He played most of the 2004 season with the High-A California League Inland Empire 66ers but played with both the Triple-A and Major League clubs in September.He made his Major League debut September 22, , playing in two games for the Mariners that season as a late-inning replacement.