A 42-year-old male on injectable naltrexone (Vivitrol 380 mg IM monthly) for opioid use disorder in sustained remission is brought to the emergency department with acute appendicitis requiring emergency appendectomy. His last Vivitrol injection was 12 days ago. He is in severe pain (9/10) and the surgical team asks the PMHNP for guidance on analgesic management, noting that standard doses of IV morphine have provided no relief. Which of the following best explains the analgesic challenge and the most appropriate pain management approach?