The only people that could possible challenge Nibbels that aint in the giro atm (or were in the giro but retired):
AC - Would probably beat him (don't know about his tt form though)
Froome - Would probably beat him
Valverde - Would challegne him, but I don't think he would have matched Nibalis tt, and he is not exactly a guy who would take minutes out of Nibali in the high mountians

Quintana - a real dark horse. Perhaps he could climb with AC and Froome, perhaps not. Perhaps he could tt like he did in Basque, perhpas not.
VDB2 - Scary

Kash - his team mate, wouldn't destoy him on purpose, but who knows
He would at worst be nr 4 this giro if Quintana is as good as we perhaps expect him to be, and ha doesnt loose time in the first week, wich would not be to unlikely.
At best he could fight for victory, like he allways do