The picture linked to shows a slightly mouse eared tile over a joist. Crawl down it and die there.
If you have a floor with a 8 x 4 The 'floorboards' go under the wall. So how to replace that bit? I'd really rather not destroy the bathroom on the other side of the wall (i t's all tiled, including the side of the bath.
Half tempted to rip the lot up and replace with proper floorboards. So what is stopping you?
***WTF are you worried about the bathroom for, you plonker?.
The joist runs along the wall about 3 inches from it; new boards will carry that far but you could screw some 2 x 2 along side it if you are afraid of it flopping down.
The wall looks like it was built over the floor as a stud wall. And the sol e plate to that will be nailed to the chipboard with 4 inch nails that won' t be doing much and are going to be doing a lot less once you chop away all the floor board. But don't let me stop you.
Your next problem will be cutting the boards flush. There are electric saws that can get at it but a small circular saw set at an angle should get the worst of it off I imagine. Just watch you don't go through wiring or plumb ing.
Next time you post a picture of a huge hole, make sure you don't post a pic ture of a mousehole instead. It will save a lot of acrimony.