Castillo has been the subject of transfer talk for more than a year, but the American appears ready to make a move, with sources telling Goal USA he's heading to Monterrey.
The 27-year-old defender has been a fixture on the left side for the Xolos, playing key roles in the team's Apertura title campaign in 2012, and their deep run through the Copa Libertadores in 2013. His form in that competition helped generate considerable transfer interest from Brazil.
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Castillo's departure during this offseason was helped along, in part, by the development of fellow American Greg Garza, who established himself as a quality left back option during the recent Clausura campaign. The Texas native took Castillo's starting job for a long stretch that included the knockout rounds of the CONCACAF Champions League. The 22-year-old Garza is a younger and less expensive option, and also a better defensive option at left back for the Xolos.
Castillo had been considered a candidate for the U.S. World Cup team, but continued defensive struggles while on national team duty cost him his chance to make the team.
The New Mexico native will be joining his seventh Liga MX team since turning pro with Santos Laguna in 2006. After winning a Liga MX title with Santos Laguna in 2008, Castillo spent time with Club America, UANL Tigres, San Luis and Puebla before joining Club Tijuana in the fall of 2012.