This is exactly what's up IMO. Sadly it is probably damaged cartilage and that stuff doesn't grow back. You'd think Buxton has done all the strengthening of the surrounding muscles to support the joint, so what else can be done? This does not bode well for his future in center field. The fact that they haven't tried him at other less demanding positions offers a glimmer of hope that they feel that either improvement is on the horizon or a second off-season procedure could provide a path to better health.
I would say the reason they haven't played him in center is because they think it could cause further, irreparable, damage.