When you have not used a joint to it's fullest capacity for a period of time you start developing what's called a frozen shoulder. Adhesions build up to prevent it's full range of motion. Those adhesions need to be worked/stretched out or you'll have a permanent issue. The physical therapy, in good hands, will work the joint passively without aggravating the rotator cuff itself. It takes time for the cuff to heal, but if it heals with too much "scar tissue " then you get the frozen shoulder syndrome. The cuff itself is made up of several tendons and, again, knowing which tendon has the tear (thus the MRI) the orthopedic specialist can help guide the physical therapist. I can diagnose a torn cuff accurately up to 90% of the time but can't say how severe or which tendon is involved. I usually refer to a specialist who concentrates on sports medicine because that's where a large percentage of these injuries come from. Sometimes, though, unless you are an athlete, it's better to avoid sports physical therapy places because they tend to be too aggressive on those of us who don't make their living by sports.
Bob