This is what alot of Jazz fans are missing when they compare Millsap and Williams. Derrick Williams is not this undersized PF who mirrors everything Millsap does. Not only is he younger, more explosive and a better finisher around the rim, but he has a bigger wingspan and standing reach than Blake Griffin, Kevin Love, Tyler Hansbrough, Al Horford, Udonis Haslem, Drew Gooden, Troy Murphy David Lee, and Josh Smith.
So let's say his transition to SF takes longer than expected or he just doesn't make it as an NBA-SF - fine, he still has the potential to be an above-average NBA PF and that makes him a pretty safe pick IMO.