Newcastle have given England striker Michael Owen, 25, until the end of Thursday to decide if he wants to join.
The Magpies agreed a club record fee of around £17m with Real Madrid for the England star on Wednesday but Owen is known to prefer a return to Liverpool.
"There is not much more we can do. We are waiting to hear from Owen and his adviser," Shepherd explained.
"But it has to happen today or it's not going to happen at all, and we will be speaking to Michael's people."
BBC Sport understands Liverpool are not willing to pay more than the £8m Real splashed out to land Owen last summer.
They are expected to reveal their hand after Friday's Super Cup game with CSKA Moscow on Friday but the player would have to take a pay cut to persuade Liverpool to do business.
Owen said on Wednesday that if he could not move back to Anfield, he would consider a loan switch to St James' Park
But Newcastle manager Graeme Souness is only interested in a permanent deal, saying: "The loan is not an option, not a chance.
"Not when we've made a whacking great offer for him that's been accepted."
And Shepherd added: "Real Madrid will not allow Owen to go out on loan but that is something between them and the player but I am sure that a compromise can be reached."