A 46-year-old man is evaluated for left shoulder pain that began suddenly 2 weeks ago. The pain worsens with overhead activities and at night. He notes limited range of motion and pain with lifting his arm. He reports no trauma to his shoulder and has never had this pain before. He is employed as a painter, and the pain is interfering with his ability to work. Medical history is unremarkable. His only medication is ibuprofen, which provides modest relief.
On physical examination, vital signs are normal. The left shoulder is normal in appearance, and there is no tenderness to palpation of bony structures. Examination reveals a positive painful arc test. Pain is elicited over the anterolateral aspect of his left shoulder with active but not passive shoulder abduction. External rotation resistance, external rotation lag, and internal rotation lag tests are negative. He has no pain with his left arm in full flexion, and he is able to slowly and smoothly lower his left arm to his waist. When the patient is asked to hold the arm flexed at 90 degrees with the forearm bent to 90 degrees (at 12 o’clock), he does not have pain with the arm internally rotated to cross in front of the body (negative Hawkins test). When his left shoulder is abducted to 90 degrees and then adducted 30 degrees in the scapular plane with the thumb pointed downward and downward pressure is applied at the elbow against resistance, he does not have any pain or weakness (negative empty can test). Strength is 5/5 throughout.
Which of the following is the most appropriate next step in management?