Thing is, Jarvan has already a good Armor growth factor (3.6) and will get an even better base value (27 instead of 24, which will be around the 15-20th rank in level 1 Armor among champions).
If necessary, Riot could even increase that growth factor further, to something like 4, if the nerf proves too harsh.
However, that being said, it IS supposed to be a nerf, after all.
And unlike popular belief, Lee Sin is, powerwise, in a good spot currently, and needs more like a redefinition of his strengths and weaknesses instead of a straight-out nerf ( position 20 of 42 junglers .)