well when rafa was in charge, toress was the lone striker there, and ngog was kinda young so was that Hungarian ST . i think for 2 years or something they didn't get involved in the transfer window in getting a new forward despite that infamous robbie keane move.
so maybe the reason was for getting a new forward to play with toress. this is as much as what i know could be the reason, but Steve might be will be more explanatory.