That's true, but Darby is playing lights out. At the NFL level, that's almost unheard of for a rookie CB. It's commonly said it has the second biggest learning curve in the NFL. Darby is playing better than GIlmore was as a rookie, and Gilmore went #9 overall.
I don't doubt Whaley thought he'd be decent, and loved his potential as a physical freak, but he's playing like a #1 overall CB. Similar to how no one thought ODB would be as good as he was once he started playing.