Need Help Repairing a SMALL Section of Diagonal Tongue and Groove Subfloor

I have a small section of flooring that was ruined (went, dry-rotted) by a leaking steam radiator. The weight of the radiator along with the moisture has caused the wood to deteriorate under a portion of the radiator, as well as a small section of the subfloor. The home was built in 1929 (in New Jersey). The flooring is hardwood (oak strip) tongue and groove over a subfloor which is set at 45 degree diagonal (also tongue and groove) over the floor joists. The damage is on the 1st floor, so I can see the subfloor from underneath from the basement. The damaged area is near an outside wall, and although the hardwood floor and subfloor extend under the wall, the damage does not. Also, the damaged area is not very large (less than 1 foot by 1 foot), and between two floor joists. I?m not sure of how to best repair the small section of subfloor before replacing the section of hardwood flooring, but I?m concerned since the (heavy) radiator sits on top of this area that it need be done securely/properly. Any advice/instruction on properly repairing the section of subfloor? Thanks in advance!

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If you are exposing the top of the subfloor by removing the hardwood, the problem is pretty trivial. Carefully cut the subfloor along the top of the 2 adjacent joists, remove the rotted boards, and replace. If you don't want to fight with the nails through that section into the joists, cut the subfloor flush with the joists, and sister a 2x4 cleat to the joist on each side, wider than the patch. If you don't want to fight withe weaving in the diagonal patch, you can just cut the patch square with the joists, add blocking below, and infill with whatever you can find of correct thickness. Sounds like an ideal job for a mult-tool with the plunge cutting blades. Cut down into joints, and cut that tongue right off.