A 38-year-old male presents to the emergency department with acute onset severe pain in the left buttock and posterior thigh following a fall onto the ischial tuberosity while playing cricket. On examination, he has weakness of knee flexion and ankle plantarflexion, with sensory loss over the lateral leg and sole of the foot. Imaging confirms a fracture-dislocation of the hip with posterior displacement. What is the most appropriate immediate next step in management?