Further tests revealed a stress fracture in John Henderson's right foot, and the defensive tackle is expected to miss three to six weeks, Raiders coach Tom Cable said Wednesday.
Henderson, regarded as one of the team's best run defenders, missed the past two games.
"It's big," Cable said of Henderson's loss. "John was playing well, so, obviously, that's a loss. But we'll overcome it."
Henderson's foot is in a protective boot, and he was spotted using crutches to get around.
Second-year player Desmond Bryant is the only other true defensive tackle on the 53-man roster behind starters Tommy Kelly and Richard Seymour.