A 65-year-old male has severe pain in his left shoulder. There is tenderness of the greater tuberosity of the left humerus. The patient is unable to slowly adduct the left arm after it is passively abducted to 90 degrees, and external rotation is limited by pain. X-ray of the left shoulder shows sclerosis of the acromion and humeral head. Which of the following is the most appropriate next step in management?