You could literally use that defense against ANY pick they made at the 8 spot. Let's say that drafted Kyle Williams there, you could still say, "PROVE they could have gotten him in a later round!" Of course the person couldn't prove it, it's impossible to prove.
The Bills did what they thought was right and hopefully it works out. That being said just about all of the info that the media and their "inside sources" have gotten from other teams say that he would have been available later. The general pre-draft grades say he would have been available later. There are no guarantees in life, perhaps he wouldn't have been available but sometimes you have to take all the info you have and roll the dice instead of locking onto the one player you absolutely have to have.
I think Whitner will be a very good player for the Bills, he seems to be a bright young guy with loads of talent that hasn't had many issues transitioning to the NFL. I do, however, believe, based on what I've read/heard that Whitner could have been had in the mid first round. It's not fact, it's just what I believe