While playing Franchise mode, your inmates can get injured after losing a fight on ice. The more brain damage they have, the more likely they are of getting injured. The types of injuries include MINOR, DISABLED and LETHAL. With a disabling injury, your inmate will leave the ice and not return for the rest of the game.
Your inmate will be placed in a bed at the sickbay, where you can go check how many days it'll take for him to recover. Giving him Cannabis will make him recover faster. Cannabis can be bought from the drug dealer outside on certain days. All you need to do is to wait and make sure you have enough money for your inmate's treatment and to keep your franchise from going bankrupt.