you can have a bunch of good to great defensive players (e.g. Rondo, Bradley, Lee, Green, Melo potentially) on the floor but KG is the type of player whose impact goes beyond the numbers. He makes good defenders great, and great defenders elite. I think it's his communication more than anything (that, and his ability to snuff out any pick-n-roll) that the team will miss the most.
i think a pretty good illustration would be if we had DeAndre Jordan instead of KG. DeAndre is more athletic, can block shots better, and is probably a better rebounder (haven't really checked stats but lets assume these are all true first). But he won't necessarily make our team better defensively, nor will he probably have a huge impact on on/off court defensive rating compared to KG.