Often, the team you are seeing for surgery will give recommendations for the type of diet to follow pre surgery as well as after.
I am in Canada and we are not required to follow a long period of dieting pre surgery but are encouraged to start exercising. Simple things like walking or more if we are capable, or adapted exercises if disabled. They also ask us to check out protein bars and shakes that we can use post surgery for snacks when we don't have time or the urge to prepare something well balanced to eat. We do follow a diet for 2 to 4 weeks pre surgery, depending on the person and their surgeon. This diet is 5 liquid Boost drinks a day plus a list of vegetables that are permitted. No carby veggies like potatoes. I was on it 2 weeks prior to surgery and lost 12 or 13 pounds in those 2 weeks. It is supposed to help shrink your liver prior to surgery to make the surgery easier.
As for protein requirements, the dietician on your team will help you calculate your needs for the day so you don't lose muscle mass.
We are advised to eat protein first, vegetables second, and carbs last after surgery. Just in case you can't finish your meal, the protein is always consumed. I wish I could give you more guidance but it really depends on your team's philosophy. If they just told you to go home and lose weight on your own without giving you a specific diet, I would prioritize protein. But if you are diabetic, you must be careful your glucose levels don't become too low. The doctor can lower your medication if needed.