Lynch suffered the injury during the Raiders' Week 6 loss to the Seattle Seahawks in London. We reported earlier today from an ESPN article that it seems like Martin takes over the lead back role while Lynch is out. It's very possible Lynch has played his final game for the Raiders, and possibly his career.
But Martin has not been nearly as effective as the Raiders had hoped as a change-of-pace back. Lynch has avoided 25 tackles this season through Week 6, ranking fourth among all NFL running backs. Martin, though, was averaging just 0.07 forced missed tackles per touch after averaging a league-low 0.06 forced missed tackles per touch last year.
Richard and Washington could see work too, but according to the story is sounds like Martin is the back to pick up for those looking for help. The story also said Lynch could go on IR and may have at least played his last game as a Raider.