I am sorry for the difficulties you have had. Your story is raising some red flags for me and a lot of questions. 1) Were the expectations laid out in advance? 2) What made the doctor choose 8 lbs? Frankly that seems almost bizarre to demand someone with limited mobility lose 8 lbs in less than 3 weeks.
Suggestions: While I have no clear answer maybe some of the following will help;
1) As someone else suggested, talk to your surgeon right away to find out his rationale.
2) Ask your surgeon to submit for insurance approval anyway and give him your reason
3) Keep a diary of all your weight loss efforts and bring to him.
4) If there is a partner in the practice maybe ask to consult with him-- (this would obviously have to be very diplomatic)
5) Drink a ton of Water and drink Protein as has been suggested-- don't be discouraged at first sometimes it takes a few days for the body to catch up. if you have Fluid retention, kidney problems, or other health issues make sure you find out if this is safe for you to do.
Good Luck. I hope it all works out.
Linda