The Sky Blues, who signed the 34-year-old from Villa on loan earlier this season, want to bring the former Chelsea, West Ham United and Liverpool man to the Ricoh Arena on a less temporary basis but are struggling to meet the player's current wages.
Coventry manager Tony Mowbray said: "There's no way we could pay similar sorts of salary levels."
He added: "I think the player will help us but there's a number of discussions to be had, not just with our football club and the clubs concerned.
"They'll have discussions with their parent clubs and after that we need to have discussions with their advisors. So there's lots of stuff to be talked about."