Provided this is what your rabbit has, it's important to be aware that the treatment with penicillin, even by injection(never orally in rabbits), can pose a slight risk in rabbits of possible antibiotic induced diarrhea and enterotoxemia. So I would be very careful about cleaning off any leakage at the injection site so it's not inadvertently ingested while grooming. I would also feed a high grass hay diet(ensuring rabbit is eating the hay well), very limited pellets, and no sugary/high carb foods. This is to maximize good gut motility and microbial balance, to help minimize the risks that some antibiotics pose disrupting the GI flora.
Benebac plus probiotics may also be beneficial to give during the treatment. Then keep a very close eye out for signs of lack of appetite and watery or pudding like fecal droppings(not the same as mushy cecotropes), which is considered an emergency.
Look at the second medirabbit link I posted. It shows photos of one rabbits progression of the disease. Severe would be progressed enough to cause significant lesions/sores, causing enough discomfort and pain to affect appetite.