Doc ordered MRI for torn rotator cuff but MRI costs too much?

I don't think I can afford it right now, it will cost $500. I don't know what to do. My shoulder seems to be getting worse, but I have a high deductable. The way it is right now I can still use my arm at work, it only hurts bad if I try to be active or try to sleep on it. I am just wondering if it is possible that it would heal on it's own? Do I need the MRI?