Ever heard of somebody washing out of the pre-op diet?
My doctor didn't require a liquid diet pre-op or any protein shakes EVER. He said they are expensive and taste bad, so he preferred you got back to eating. I was on mushies the day after I came home from the hospital. I'm 4+ years out and have never had a protein shake....I've had a bar or two because there are some I really like and they are convenient when traveling.
Call your doctor. There has to be alternatives.