A 43-year-old morbidly obese man is undergoing a Hartmann procedure for Hinchey IV perforated diverticulitis. Two days postoperatively, the ostomy has retracted below the skin and appears purple. The patient remains afebrile and hemodynamically normal without peritonitis. Intubation of the colostomy with a test tube reveals black mucosa proximal to the fascia. What is the next step in management?