If you eat much less calories than necessary, e.g. 1000 kcal/day or even less, you will also lose muscle mass.
However, if you do not eat enough proteins, you will lose muscular mass and the albumin amount in blood will diminish, regardless how many kcal are provided by your food.
So for preserving muscular mass, both enough proteins and a minimum intake of kcal per day, which varies between people, but it is typically somewhere between 1500 and 2000 kcal/day, are required.
Nevertheless, usually only the amount of proteins requires special care, because it is extremely easy to eat enough kcal per day when you eat enough proteins. Normally a great effort is required to diminish the amount of kcal of the food that contains enough proteins, especially if it is of vegetable origin or of dairy origin.
The only natural source of proteins without extra kcal is lean meat, like turkey breast or chicken breast or cod fillet. Fat meat, most dairy and raw vegetables have a kcal content that is usually more than double of what corresponds to their protein content.
For example, when the easy method of eating lean meat is excluded, enough proteins for a male of average size could be provided by eating 500 g of wheat flour and 167 g of lentils per day. That alone would provide 2235 kcal/day, which is too much for someone with a sedentary life.
Preserving the amount of proteins and also allowing the eating of other food besides that, without using industrial methods of protein extraction that are expensive and require energy and chemicals, could be achieved by making dough from the flour and washing it to remove most of the starch.
This example is meant to show that unless you eat only turkey breast or the like, as long as you eat enough proteins you normally have no need to worry about eating enough kcal.