Be careful when alluding to those College guys as "Play right now". We have seen time and time again that many can't make the jump to the NFL tier right away. Just look at Lattimore. He couldn't stay healthy on the college level with hamstring problems. NFL training, practices and games are much more rigorous then anything they have ever done.
CB in particular is a position you should never have high expectations for a year 1 contribution. Speed of the game alone takes years to get used to, and a lot of your first round corners end up playing as the #3 or #4 corner in nickel/dime packages for much of their rookie year. I would have zero expectations that a 1st round corner we take is going to line up outside and start covering quality WRs by himself.
You are right about that. I was high on Sidney Jones, as a CB that I was hoping would fall to us in the draft, until the Achilles injury.