If your dog eats grass it is not something you should worry about. Modern dogs are not like their ancestors, which means that do not adopt eating habits of them, such as eating a prey entirely.
Experts say that modern dogs use to eat plants (mostly grass as it is the closest at hand) because of evolution and domestication. In other words, dogs eat grass as an alternative food source.
Apart from grass, there are dogs such as wild canines who eat fruits, berries, and other vegetables.