The egg is necessary for making a cohesive meatball. Without it, your meatballs will fall apart. But all is not lost if you don't have an oval friend in your fridge. Go with ground flax seeds, chia seeds, mashed bananas, applesauce, or silken tofu. To use any of these substitutes, simply mix the ingredient into the meatball mixture in place of the egg. Keep in mind that each of these substitutes will slightly change the flavor and texture of the meatballs, so you may want to experiment with different options to find the one that works best for you.