Disclaimer: I'm not a Doctor or any kind of medical practitioner!
Basically as I understand it, these abnormal bone growths are in a place that they shouldn't be, i.e. the muscles. The more mechanical movement you do, the move damage that the bone growth does to the muscle through friction. Once it gets so bad to cause enough pain and discomfort to prevent you from exercising, you have to simply let it heal and the process starts again. Of course, weakening your thigh muscles is not a good thing for the knees generally as well, i.e. ACL injuries.
Take this with a pinch of salt. It is simply my layman's understanding.
I honestly know nothing about footballer's having this kind of surgery. The size of the object to be removed, will it grow back, impact on the surrounding muscles. All I know is that Moyes was talking it up as Gibson kept breaking down and would require recoveries of various lengths of time from 2-12 weeks or so.
Also possibly tangentially related (abnormal growths) is the fact that Gibson was injured against Scotland when a cyst at the back of the knee burst. I believe he then suffered setbacks following this which kept him out for the following 3 months until he returned in February.