Castillo broke his left leg in the 2011 season opener. He was placed on injured reserve on Dec. 8. This will be his eighth season.
The Chargers also signed former Tampa Bay wide receiver-kick returner Micheal Spurlock to a one-year contract. The Chargers will be his fourth team.
Spurlock is one of only two players in Tampa Bay history with both a punt and kickoff return touchdown to his credit. He was the first Buccaneers player with two kickoff returns for touchdowns.