After beginning his career with native Club Atlético Platense, 21-year-old Díaz moved to Spain in 1997, going on to remain in the country the following 12 seasons. Two years earlier, he was picked to represent Argentina at the FIFA U-20 World Cup, with the national team winning the tournament in Qatar after defeating Brazil 2–0; during his career, he was nicknamed "Camioncito" (Little Truck).