Same thing that was wrong with him for the last couple of years -- he's an old, slow power back (just like The Bus -- love him, but his bad days will far outnumber his good for the remainder of his career).
As for dropping a starting RB for a 2nd stringer, I'd have to be pretty sure that Edge isn't coming back before I'd drop a starter for Little Ricky. In a couple of weeks, you might be regretting that move if Little Ricky is parked on the bench...
QB: A. Brooks; P. Ramsey
RB: C. Portis; T. Henry; M. Williams; L. Gordon
WR: J. Horn; L. Coles; K. Robinson; C. Rogers
TE: A. Crumpler; D. Clark
K: S. Janikowski; O. Mare
Def: Panthers; Falcons
Start: QB, RBx2, WRx2, TE, K, Def