That doesn't look all that bad for a possum attack, they are clumsy stupid predators. My Mother In Law had a hen get attacked while she was roosting at night. Luckily the possum couldn't get a good hold on her and she only had some superficial gashes on her back/hip . She was cleaned up, sprayed with blue kote, and healed just fine. Just make sure you disinfect the area with iodine or blue kote so it doesn't get infected.