That was the response I was expecting, but hoping I wasn’t right.

That’s cement tile backer board you see. I did the same thing to my shower a few years ago if it makes you feel any better. Where on the shower is this? Can you take a wide shot, please? Depending on where it is, you might be able to patch it for the short term.

BS. It looks like it's not in the wettest part of the shower. You seem to have broken the cement board behind. You could try to patch in a piece of cement board, after cutting out the bad stuff. Then scrape and apply some new mortar. On the other hand, I would be tempted to use mortar and a roll of mesh to replicate the cement board, removing loose chunks of cement board, but leaving the mesh in place. Either way, you'll have to pay attention to making a flattish surface for the final application of mortar and tile.

Couple wider shots. I obviously stopped removing the other tiles, but will still have to remove them so I can put full size tiles back. Will try and be even more careful when removing, but the first tile was stuck onto the mesh and cement pretty good.