Subfloor damage under exterior wall
It's difficult to say from a photo but it appears that there is damage to the piece marked below.  It's also quite possible that there is more damage on the other side of this wall that you cannot see now.
I'd tear the whole thing out and fix it right.  Of course that's a bit of work and it depends on whether you are looking to fix it for the long term or just wanting to patch it back together for a short term fix.