Many of the underclassmen, including Jadeveon Clowney and Johnny Manziel, are among the highest-rated prospects available this year, and the juniors and redshirt sophomores that declared will be getting a particularly close look from teams at the combine. Given their shorter careers, there's less game tape available to evaluate than there is for seniors, and that means more work during the draft process for general managers to find out just what they're getting in a younger prospect.