"Wide receiver Vincent Jackson almost certainly will not play Sunday against the Oakland Raiders and could be out longer with a strained right calf suffered Sunday in Indianapolis.

Jackson hurt his left leg in practice Friday, though he did not come off the field. He then injured his right calf warming up prior to the Chargers' victory over the Colts. He played two snaps before exiting the game and not returning.

An MRI on Monday morning confirmed the strain, and head coach Norv Turner said Jackson has substantial swelling.

"I would think it would probably be hard for him to play this week," Turner said. "Then, after that, it would be on a week-to-week basis. We will see how he recovers. I may be premature in saying it will be hard for him this week, but I want to be realistic."

A source, who did not want to be quoted because he was not authorized to divulge injury information, said Monday morning that Jackson tweaked his left calf in practice Friday and then injured the other calf Sunday, likely by overcompensating for the original injury. Another source, who also did not want to be quoted because the team does not generally release specific injury information, said Jackson was sore Friday and "he did more than he should have" in the lead-up to his first game.

Reports by other media outlets late Monday, citing sources, said Jackson's injury last week was to his groin but also confirmed the injury Sunday was due to overcompensating for the original injury."