How about this for a thought: Westbrook's value doesn't go up.
If the offense suffers with McNabb out, I think the WHOLE offense suffers. I'm particularly dubious of the argument that Westbrook's value increases because of the fact that he catches so many passes. Short passes, yes, but short passes are, sometimes, a luxury that a struggling offense doesn't have. It's run, run, and pass for 3rd and 8.
Westbrook's running may not be so effective either if opponent's don't fear the deep pass.