Kwamie Lassiter, who signed as a free agent from Arizona in the offseason, tore the medial collateral ligament in his right knee in Sunday's 37-8 loss at Denver.
He was supposed to be the glue that held the San Diego secondary together but he did not provide the over the top protection needed for the young corners.
Lassiter was second on the team in tackles and rookie Terrence Kiel will now start.
The Chargers signed cornerback Tony Okanlawon, who had been released by the team on Aug. 31 as part of the final roster cutdown, to take his place on the roster.
Tim Dwight is expected back from Denver late Wednesday, where he'd been hospitalized since sustaining a partially collapsed lung on Sunday.
"He's not going to be able to play this week," Marty Schottenheimer said of Dwight.
Ray Lee Johnson will also not play this weekend and he may miss a few games.
Wide receiver David Boston didn't practice because of a shoulder injury.
Chargers Place Lassiter on IR
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