Dogs and cats sometimes need a special diet when they are recovering from an illness or surgery. Your vet will often recommend a diet that is high in protein. Before you jump to the conclusion that meat is the only food high enough in protein that a dog or cat can eat, think again. Their digestive system might be compromised by their illness so meat would be too hard to digest for them. That is when you need to look at other forms of protein.
One very good source is cottage cheese. It is high in protein and very easy to digest. Check with your vet to see if they recommend a high or low fat content type. Another source of protein is yogurt. Of course the plain version is the one to use, which incidentally is the best acne treatment for humans. Both the cottage cheese and the yogurt can be mixed with enough canned pet food to make it smell more appealing if your pet does not show interest in either one of them.
Eggs are another good source of protein. Some people feed the eggs raw to their pets but with a pet that is weakened it is best to cook the eggs first and mix them with something else that is easy to digest.
While soy is a high protein source it is also a plant, which means that the individual cells of the soy have walls like all plants do. The walls make it harder to digest compared to animal sources of protein which do not have cell walls.
When thinking about easy to digest sources of food for an ailing pet, think in the terms of cell walls. You want to have a diet that does not have plant cells in it. Plant foods take longer to digest and their nutrients are not readily available to the pet for 12 or more hours.