Castillo injured his tibia in the 2011 opener and missed the rest of the season.
The Chargers drafted him in the first round in 2005. Castillo started 79 games and had 19 sacks and 43 tackles for loss.
The Chargers also say they will not use the franchise tag this season.
The Chargers tagged wide receiver Vincent Jackson last year. But the franchise designation would have required the team to offer the seven-year veteran a contract at 120 percent of last year's figure. That would have carried a salary-cap figure of about $13.7 million.
Free agency begins March 13.
General manager A.J. Smith says the team would like to keep Jackson "but other teams now will enter the picture."
Information from The Associated Press contributed to this report.