A 52-year-old woman from rural Maharashtra undergoes an uncomplicated total hip arthroplasty for osteoarthritis. On post-operative day 1, she complains of severe pain in the left calf and foot, with inability to dorsiflex the ankle. Examination reveals foot drop, weakness of ankle dorsiflexion and toe extension, with sensory loss over the dorsum of the foot and lateral leg. The surgical site appears clean without signs of infection. What is the most appropriate immediate next step in management?