 In 1977, Valenzuela began his professional baseball career when he signed with the Mayos de Navojoa.A year later, he was sent to the Guanajuato Tuzos of the Mexican Central League, posting a 5–6 record with a 2.23 ERA.The following year, the Mexican Central League was absorbed into the expanded Liga Mexicana de Beisbol (Mexican Baseball League), automatically elevating then 18-year-old Valenzuela to the Triple-A level.