McNabb takes a small hit. Overall offensive efficiancy goes down a bit without Westbrook. McNabb's numbers may or may not go down, depending on the nature of the opponent.
Against a team like Chicago, I'd say his numbers take a small hit. Efficiency will go down, but Philly's defense s likely to throttle Chicago, so Philly stays conservative with a lead.
Against a tougher team (Giants/GB/Dallas types) McNabb would throw even more then normal, and make up for lack of efficiancy with more attempts. Philly has not had good success against good teams without Westbrook, but McNabb has continued to produce decent fantasy numbers.