True protein is important but you cannot maintain muscle mass by eating protein you have to generate it physically with weight training.Also very important that when on a caloric deficit it's essential to eat enough protein to maintain your muscle mass, otherwise your metabolism slows down even more and you burn less calories.
Always cut calories from carb and fat intake first.
If you don't use it you lose it, regardless of how much protein you have, and then you still lose it.

