Santos fell last weekend in the semifinals of the Apertura 2008 championship. Castillo scored in Santos' 2-1 second-leg loss to Toluca.
After completing high school in Las Cruces, N.M., the U.S.-born Castillo attended a Santos open tryout and was offered a long-term contract. Castillo made his First Division debut in April of 2006 and won a starting position in January of 2007.
The left back has been hailed as one of the Mexican league's best young players and he debuted for Mexico's full national team, for which he's eligible because of his Mexican heritage, in August of 2007. He also played for Mexico's U-23 team in its failed bid to qualify for the 2008 Olympic Games.