It sounds like you will have to cut out the entire damaged section. That wouldn't be a bad idea anyway as you want to make sure the studs are not damaged also. FWIW, many campers ago I tried to fix water damage behind a cabinet without removing the cabinet. I ended up removing the cabinet anyway. I was able to get by without removing the countertop. It made the repair easier since I had better access. As for trying to match it, I wouldn't try. Perhaps peel and stick tile or something similar along the entire area under the cabinets would work.