I do not think this hypothesis is correct. What evolutionary purpose would functional pain serve?
To me, it seems it's a theory to compensate for medical imaging technology that has far too low resolution. Doctors hate admitting that they don't know what's wrong, so instead they claim that the patient is crazy in politically correct terminology with an excessive number of syllables.
Structural tissue damage starts at a molecular level that cannot be seen by medical imaging technology. The fascia alone is too thin to be imaged at all, and so is completely ignored.
Biochemists have delineated in explicit detail how tissue breaks down at the molecular level. And they can reverse it. We need to start listening to them more.