I had a hen that was cut from her back to below her thigh-probably 4" long and deep. I did not find it until several hours after she was attacked (probably a hawk). The skin was to dry and to far apart to sew up. I washed it in betadine and water, packed it with antibiotic ointment. I kept her in the house in a pet carrier for a couple of days. I just watched it for infection and applied ointment for several days. I did not put a heat lamp on her since she was inside. I was amazed at how the wound healed. She grew skin and feathers back over that area. The scabs took care of themselves. It took several weeks for it to heal completely. (The bruising looked almost green but was not infection.)
Since this happened, I will not panic when and if one gets a bad cut again. I will clean, apply antibiotic ointment and watch for any signs of infection. If I see any signs of infection, I will start an antibiotic immediately.
Good luck with yours.