Assessing water damage to wood beneath bathroom
I've seen a lot worse. There doesn't appear to be any mold. I'd rip the thinset up to the extent of the loose tiles and the water staining below as soon as possible and start the drying out process. When the drying is complete, you'll be in a better position to judge for damage. If you caught it fast, you'll probably be OK. Once dried, you might think about spraying some white vinegar on the wood to kill any mold/mildew that might be thinking about growing.