Preheat the oven to 350 degrees F and grease a 9-by-13 casserole dish.
Using a sharp knife, cut out the top of each roll and remove enough bread to fit a meatball inside. Place rolls in a casserole dish as they are done and set aside.
Place the meatballs and marinara sauce in a medium-sized saucepan set over medium heat. Cover and cook just until sauce and meatballs are both hot throughout.
In a large frying pan set over medium heat, melt butter. Add garlic and salt and saute for 1 to 2 minutes, just until garlic is soft.
Using a pastry or silicone brush, brush the garlic butter generously along the inside of each roll.
Place a meatball inside each roll.
Top each meatball with a tablespoon or two more of marinara sauce and sprinkle with mozzarella cheese.
Place the casserole dish into the preheated oven and bake for about 10 minutes, just until cheese has melted.