The previous owner left me with several gifts. This being one.
I had planned to lift out the toilet, cut out the section of the sub-floor that is damaged, then put a few 2x4 on the flat between the joices to support a new subfloor I would install ...lastly I had planned to properly install the toilet.
Any other advice?
Just FYI, I think the previous owner put down a new subfloor to instal new tile and never properly re-installed the toilet (when he lifted the floor) .. but that is just my guess... I will know more once I lift out the toilet.