I don't think Djokovic withdrawing from this match will be of any benefit. If he plays, clearly can't win, and is risking making his injury worse, he can always just retire mid-match. Not an honorable thing to do, but it would just be stupid to not give it a try. He might feel in form tomorrow, and I'd rather people see Novak winning Cincy than see him withdraw. It'd get further in their head, rather than see him as vulnerable they'd see him as the guy that won the first event coming off a huge break and the owner of a 20+ match win streak.