I would firstly like to point out I have no personal knowledge of either guys injury - just my professional view from what I have heard.
The Andrew Johns one is interesting. They never actually said (or I never heard) what the injury was in the end, though this
was the problem in the past. One would assume that the disc injury became worse and possibly starting to cause spinal cord compromise.
The problems Nick has been having (for as much as I know about them) sound not disimmilar in that there may be a disc problem. This was my feeling yesterday, and generally, minor disc bulges respond well with rest and special exercises, but (as with last year) you can carry on life.
More serious bulges can become like Johns.
My gut says that they have the scan to show the bulge, one specialist has said you'll need X time off......or retire..........and they are going for a second opinion.
IMO you would only go to Sydney for a second opinion if it was the difference between playing and retiring. Don't think they would go up their if it was to decide between 6, 10, or 19 weeks.
As I said, I hope I am wrong (happens occassionally ), and all the best to Nick and his family.
Cheers BB