Hello and welcome. My name is Scott and I am an experienced small animal veterinary surgeon, I will be very pleased to work with you today and will try my best to answer your question to your satisfaction.
The situation is this : Some cats have cruciate ligament injuries and then they quickly recover over the course of one or two weeks without any intervention. This is no problem ...
Other cats have more severe cruciate ligament problems and remain lame, the best option for these cats is in fact surgery.
You see if it is not tackled fairly quickly there is a risk of severe arthritis in the affected leg later in life.
The bottom line is that I would agree with your vet that surgery is probably your best option.