I actually found this quote to be more telling Vinny:
"So much of playing the position is skill, instincts, knowledge, all of those things.Just because a guy can run fast and has size doesn't mean he's going to be a guy who is going to go out there and shut a guy down."
A young CB can come in with the skills and instincts but they will lack the knowledge. I think it is a position where you basically "have it" or "you don't" in the long-run. I cannot think of a position other than QB where you learn as much through trial and error. A CB may be able to step in and make plays immediately, but he is also going to make enough mistakes to counteract it. Just look at Dunta's season so far. It is a blend of big plays made and made against him. I think that is normal for talented young cornerbacks.
There is no doubt in my mind that RB is the easiest position the field to step in and play. The only slow learning part of that position appears to be blocking, especially blitz protection. Otherwise it seems like a quick transition.