Well, as it turns out, the story was only half-right. It's true Knapp is no longer able to call plays, which is probably a good thing based on recent results, but it wasn't Cable who stripped him of the ability.
It was Al Davis! From Nancy Gay of the San Francisco Chronicle:
"Knapp just happened to be the next in line to be a scapegoat," said one source, speaking on the condition of anonymity but eager to clarify who stripped the play-calling duties from the offensive coordinator. Under Davis' orders, Knapp had been calling plays since shortly before head coach Lane Kiffin was dismissed Sept. 30.Oh, Al. Everything else remains the same, of course, deck chairs and what not. And I feel the need to point it out again, but, Jim Harbaugh ... stay away! Far away!