يونيون إسبانيولا
Sierra made his debut in professional soccer with Unión Española on November 1988 against the Universidad de Chile. After two solid campaigns with Unión Española, Sierra was transferred to Real Valladolid in Spain. However, during Sierra's short time in Spain, the team had economic problems which caused him to return to Unión Española. After more success he made his debut with the Chilean national team. Sierra won the Copa Chile with Unión Española in 1992 and 1993.