Wet wood needs to be properly dried out from ABOVE. The dishwasher and cabinets all need to come out soon or you will have mold.

You have to remove ALL of the water damaged wood first. You csn’t repsir snything until that is done. Start excavating.

I sure dont like the looks of that electrical wiring sitting there all wet either. I agree. It may cost time, and it may cost money, but, removing it all is something that should be done, and it needs to be made safe. That electrical there, doesnt look one bit safe to me. Safety is always a priority thing. It appears there is also a bit of mold there, and thats not a good thing either.

You don't have to replace a 4x8 sheet of subfloor. You can replace the material that is damaged between floor joints.

@Lomo @millworkman @cat_ky As mentioned in the post, this is a photo taken when the leak just happened. Now the wood has been dried out already. What should I do with the falling piece as the next step? Thank you.

@dan1888 Would you please elaborate more on how to do that? Thank you.

Be aware, just because it is now dried out, does not mean the wood is not damaged, and in any case, the electrical in their where there could be a water leak again, is dangerous. Safety is always priority.

Stick an awl into the wood in several places to see if it's damaged or just discolored please.

HU-874248819 The subfloor is nailed and glued in many cases to the joists below. These joist are 2x10s or similar with 16 or 24 inches of space between them. You can cut and remove a damaged portion of a 4x8 sheet of subfloor that runs between two joists. Remove to the middle of a 1-1/2" wide top of the joist. Sistering an added piece of 2x to the joists can also help provide support for the new piece of subfloor. Adding a 2x4 gives another 1-1/2" width across the top of the joist in that area. Test for damage like Joe mentioned. Test the subfloor and the joists.

Watch videos from YT on installing subfloor to get a better idea.

Another for older flooring.

Joseph and Dan are giving you the correct advice. Take out the badly damaged wood and replace only as much as is required. Mold is not an ongoing issue if you remove the water source. Sistering the joists to create a nailing surface is way easier than trying to cut the old floor exactly over the middle of the joist.