Below average (McK) > Terrible (A.Williams).
McKelvin has this advantage: He is able to stay in the area of the receiver and appear to be covering him. True, if the QB actually throws the ball there, McKelvin is unlikely to break it up. However, his relatively close coverage may cause the QB to hesitate or check down instead of throwing the ball, because he may not always realize that the "coverage" is partly an illusion. Compare that to a CB who is badly out of position; the QB sees green space around the receiver, or a Bills player actually running the wrong way, and throws immediately for a reception.