Body Damage to My RBP

I need help. I left my house for Christmas break and came back a week and a half later to find my smallest RBP with a large gash out of his back. The other 2 RBP in the tank are find, but im worries about this little guy. (3"). I have left them before for a week and nothing had been wrong with them, but not that kinda luck this time. He's missing a good portion from the back of his skull to his top fin. Im wondering if this is going to grow back seeing as its not a part of his bones. I know there will probably be some long term damage, but should I be really worried about his back not properly growing back?

Thanks for any suggestions or help.