It appears that the Giants are one of 3 teams (Rams, Bengals are the others) that don't have a 3rd QB on the roster or the Practice Squad. I think that under Coughlin they have typically had 3 QB's on the roster or a practice squad QB (e.g. Ryan Perriloux).
A practice squad QB could be some developmental prospect, someone who can simulate an RG III type QB in practice, or a Green Bay castoff who knows the system. (I was thinking Scott Tolzien but Green Bay kept him).
The Giants could just have 2 QB's and no practice squad QB. It seems certain that Curtis Painter would be a phone call away if anything happens. Perhaps another possibility is that they put Painter back on the roster after week 1 so that his $$ are not guaranteed (might provide more cap flexibility).
Nassib can probably run the scout team fine. He's still learning and the reps will do him good.
If the GIANTS don't, they'd better hope they don't need Painter because with the state of the QB situation in places like Houston, Buffalo, and maybe Oakland, and considering that some other teams only have one solid guy and he could go down, even Painter might not be available.
Of course, if Reese & Company hadn't let the OL go all to hell, this wouldn't even be a thread at this point. But that's life in the fast lane.