The Jags official site reports on the team moving Reggie Nelson from safety to corner:

Reggie Nelson moved from safety to cornerback to replace the injured Rashean Mathis and Nelson was targeted by the Titans, in both the passing and running games. “Yeah, he had a tough day. He’s one of the guys who struggled with the tackling part of it. For him to continue to play, he’s got to play better. He understands that,” Del Rio said.