I think most of the reason for him not getting scooped up is injury history, and making sure he is fully recovered from his knee injury before inking anything.
As for talent he has enough to be a starter in this league... when healthy. When the Lions actually committed to running and gave him 20+ carries I'm quite sure they had a much better record than when he got <20 carries. Sorry no stats here to back that up.
A stat I track is career fantasy points per game which will give some insight to NFL relevance and removes injury from the equation. Looking at all RB's drafted in 1999 and later, he is #22 in fantasy pts per game with 10.7 - better than Mike Anderson, Thomas Jones, Travis Henry, Deshawn Foster, Tatum Bell, Marion Barber, Ryan Grant, Lawrence Maroney, Lendale White, among many others. Alot of these guys had better individual seasons, but not better career pts/game which tells me KJ can be a factor WHEN HEALTHY.