Castillo broke his left leg in the season opener on Sept. 11 and the Chargers kept him on the active roster in the hope he'd be healed toward the end of the season.
Triplett spent seven weeks on the Chargers' practice squad in 2010. Triplett signed with Indianapolis last December and played in the Colts' final five games on special teams. He also appeared in Indianapolis' two playoff games. Triplett played in three games this season for Indianapolis.
The Chargers also signed defensive end Damik Scafe to the practice squad.