Normal AP Shoulder X-ray

A 28-year-old male athlete presents to the emergency department with severe left shoulder pain after a collision during a football match. He reports his arm was abducted and externally rotated at the moment of impact. On examination, the shoulder has a "squared-off" contour with loss of normal deltoid roundness, and the patient holds his arm in slight abduction and external rotation. An AP shoulder X-ray is obtained. The structure marked **A** (humeral head) is noted to be displaced antero-inferiorly. Before attempting reduction, which of the following is the MOST critical clinical action to perform?