At that point, I was able to cut out and replace the center 18" of plywood and sound board and replace both. (It was mainly the soundboard that required replacing.)
|center of floor removed|
In my case, I also needed to add 2 additional 2x4 supports (bolted to the frame) the length of the frame to support the new soundboard and sub-floor. Once installed, I used a self-leveling compound to bridge the old and the new and prepare the sub-floor for the new tile.