I don't know whose ratings of Green's defense you've been reading but that recovery block of his against Curry mid-chuck would be the zenith of any defensive specialist's resume in this modern, chuck-happy nba. Can you name any other player who has stuffed a Curry 3 as textbook clean as Green did? the best I could do in pick-up games is occasionally plant a fingertip on a released ball. Green's feat needed extensive scouting, exquisite timing and cat-like reflexes. He probably counterfaked Curry into thinking he fell for the first fake when he wasn't completely off-balanced as he first appeared, so add IQ to that. See, the fact that we are not even sure what fundamentals went into that block means there's not enough words said and written about said block, so by definition it's underrated.
https://www.youtube.com/watch?v=C27yHs3FJ0k