The MRI on Oakland Raiders quarterback Bruce Gradkowski revealed a third-degree separation of his throwing shoulder, Steve Corkran of the Oakland Tribune.

Third-degree separations of the shoulder are the most severe, and require surgery and several months of rehabilitation.

Gradkowski was named the starter the night before Sunday's 33-17 loss to the Miami Dolphins. It was Gradkowski's fourth start of the season, and his first since aggravating an injury to the same shoulder in Week 5.

A favorite of head coach Tom Cable, injuries have kept Gradkowski off the field throughout the last year. Two torn knee ligaments ended his 2009 season, and the former Toledo standout suffered a torn pectoral muscle and strained groin during the off-season, keeping Gradkowski from seriously challenging Jason Campbell for the starting job.

Campbell will likely start against the San Diego Chargers on Sunday. Earlier this season, Campbell replaced an injured Gradkowski to lead the Raiders to a 35-27 win over the Chargers in Oakland.

In his first season with the Raiders, Campbell has completed 54.7% of his pass attempts for 1,322 yards with 7 touchdowns and 6 interceptions, adding 101 yards on the ground.