A 17-year-old male high school baseball pitcher presents to your office complaining of posterior shoulder pain and weakness. His father says his coaches have reported a decrease in velocity as of late. On clinical examination he has an abnormal O’Brien’s test and weakness in external rotation with the arm by his side. He has 5/5 strength with empty can testing. Radiographs do not show any abnormalities and MRI is seen in Figure A. Based on his diagnosis, which muscle would be affected and at risk for atrophy without intervention?