Well, you have to pick your poison and given the option of taking a man out of the box (e.g. playing Nickel) and letting Johnson running free or playing it straight up (7 men) or bringing a Safety down close to the LOS and make a new QB try and beat you, I know which one I'd pick.
Purdue feels real good about both starting CBs and isn't afraid to let them play one-on-one. They'll likely have to a good deal of the time if they are going to have any chance in this game. When they do play Nickel, they'll probably drop one of the CBs down to cover the slot receiver.
From what I've read about Birdsong is that he has a great arm but doesn't quite have the touch on a lot of passes that, say Cato, had. I'd also be surprised if he tried to run as much as Cato. He seems like more of a pocket passer to me. Certainly would expect the jet sweeps, presumably with Foster, who's a talent.
Birdsong is much more of a runner than Cato.. Cato learned to run because it was a necessity to the offense since we employ the read option pretty frequently.. However, Birdsong is a more natural runner.. He has a big, strong frame and deceptive speed.. He ran for over 300 yards as a sophomore at JMU where he was sacked more than 30 times and also didn't play in a offense that utilizes the spread in the fashion that we do.. So I look for Birdsong to be much more of a running threat than Cato ever was.