Water damage help

Pulled up carpet to install tile and found some water damaged plywood. It appears to be old as it's not currently wet and it's not soaked through. Can I sand it and treat it? Do I need to replace it completely? Any help or suggestions would be appreciated. http://ift.tt/1i9NLOO

Submitted August 29, 2015 at 08:32PM by TheVanDSM http://ift.tt/1F6xyyI