Athleticism and lateral quickness are not one in the same. Case in point: Jefferson. He's still possesses just a cut under elite level athleticism, but he lacks exceptional lateral quickness and because of that and the fact that he's clearly developed some bad habits over the years playing on losing teams, he's just a mediocre defender at this point.
You have to wonder, how much homework did the Spurs do on this guy? Or did they just think to themselves "we're the Spurs, under our tutelage and the fact that he'll be excited to be on a relevant team again, he'll instantly be a plus defender again"? I wonder, because Pop first talked about Jefferson being the stopper upon his being acquired, then by the fourth game of the regular season he already gave up (rightfully so) on that and hasn't reverted to it? That's telling.
The lack of a second legit big man who can start and finish next to Duncan and (it doesn't have to be an All-Star, just someone of starter quality) at least be average in most facets of the game is beyond glaring at this point. Pop would rather play small, tax Duncan and lose in the process rather than finish with McDyess, who was presumable brought in to do just that. Again I say, that's telling.