Is any RB worthy of a first round pick?

According to a recent article at the NFL Stats blog, they're worth their weight in gold. According to the article, a running back's career yards per carry and likelihood of attending a pro bowl are both strongly correlated with their draft order. Many have argued that Shanahan's selection of undrafted and late-round running backs are evidence that he has a penchant for finding late round steals. The author of this article notes an interesting point:

"For any one RB's season, offensive line ability has a tremendous influence on his stats. But over 490 careers and over 24 years of data, the abilities of offensive lines will average itself out to a great degree, leaving career YPC a reliable estimate of true RB performance when grouped by round or draft order."

Using the same logic, you could inductively argue that a running back YPC can be bolstered by a great offensive line. Anecdotally, there's really only two "great" running backs that have come out of Shanahan's drafts in recent years--Terrell Davis and Clinton Portis. All other backs failed to produce similar numbers behind different offensive lines. Is it possible that Davis and Portis were merely the offspring of serendipity?

In the event that we pass on the top runners available in this year's draft, I hope not, for Shanahan's sake. After amassing a paltry 428 carries last season, perhaps it's time to make a high-percentage investment in a workhorse back.