I'd predict that the RBs likely to outperform their current value the rest of the year, regardless of injury status to the starter in front of them are:
Wells
Donald Brown
Mike Bell
McCoy
Shonne Green
McFadden
Forsett
Essentially, many of these guys are rostered so this becomes a buy low list. I think that each of these guys are decent buys right now. Forsett is probably the only one that can be had for merely a bag of chips, but they each are in situations in which they could be regular fantasy starters down the stretch.
Mike Bell - Saints O, decent schedule, PT always leaving the door cracked being dinged up, Saints using 3 RBs at times last year. Hardly under the radar, though.
Danny Ware - He's in the position Bradshaw was last year behind Jacobs and Ward.
Justin Forsett - JJ doing nothing much, Edge washed up pretty much.
Kolby Smith - LJ is miserable and Charles can't even displace that, so maybe Smith can recapture some magic from his pre-PUP days.
Bernard Scott - Cinci run game is good; Scott probably wouldn't be as good as Benson but still could provide nice numbers.
Shonn Greene - Needs an injury.
Chris Wells - AZ hardly runs but he did have a couple more carries than Hightower last game.
Brandon Jackson - Always hoping for something but doesn't usually deliver.
Chester Taylor - As always, big if AP is out, but usually sits rotting on fantasy benches otherwise. Not exactly under the radar but not always thought of immediately, either.
J. Norwood - Has to stay healthy and needs a Turner injury.
Melwelde Moore - Would need a continually-hobbled FWP and Mendy to go down.
And now some really off the radar guys ...
Michael Bennett -- Not that great but LT can't stay healthy and Sproles hasn't been able to supplant LT so far.
Chris Henry (Tex) - Chris Brown doesn't do anything but GL carries and doesn't do that well, Slaton is doing ok but probably needs someone to spell him a bit, Moats really isn't the guy, and Henry flashed some potential with Tenn.
BJGE - Needs Maroney and Morris to get dinged, which is possible.
Samkon Gado - Needs a SJax injury.
Peyton Hillis - Would take Moreno and Buckhalter injuries but showed he can do something if the opportunity arises.