Teams learn the hard way that while you get the occasional Tom Brady, Kurt Warner, Russell Wilson later in the draft, if you go in planning on getting your stellar QB in later rounds, you are going to be disappointed.
Same thing with LTs. You can convince yourself that you can take a "specimen" later in the draft and train them up to be a really good starting LT; but that just doesn't happen very often. So it's a bad plan to believe you can solve the problem that way.
It's a little like making your retirement plan as winning the Lottery. It's not impossible; but is foolhardy and highly unlikely to succeed.
This is why I say that if a starting LT prospect is there at 23, the Rams should jump all over it and thank their lucky stars. Because now they have a back-up plan, and at least a year to train up a guy to eventually take over for Whitworth.
