I’d never heard of an avulsion fracture. WordPress spell check doesn’t even recognize the word. Anyway, according to Wikipedia, it is when a small piece of bone gets ripped off when a muscle is flexed beyond the bone’s breaking point. Basically, the ligament or tendon is stronger than the bone it is attached to. Goeido was diagnosed with such a fracture in his right shoulder and has been told to get 10 days rest.
This Timberlane Physical Therapy website has a lot of information about shoulders and goes into quite a bit of detail. It’s a pretty interesting read.