Derrick Johnson was brought in this summer to be a leader on this defense, especially when NaVorro Bowman was not re-signed by the Raiders. Johnson has played a big role on the team through the first four games, but saw his snap count shrink to 18 on Sunday against Los Angeles, and for good reason.
According to Pro Football Focus, Johnson was one of the lowest-rated players on the Raiders defense this past weekend against the Chargers in terms of tackling, and that is saying something. He has not been the player the team had hoped he would be when they signed him, and may see his snaps reduced if he does not start to produce.
I understand Johnson is old, and to expect him to be a physical leader on this defense is probably asking too much for someone his age, at this point in his career. The Raiders would be better off letting some of the younger players play in his place, as they try to actually get some stops this weekend.