McCall confirmed Murphy was due to be holding discussions with another club on Monday and that the Blades were front-runners to sign the forward.
But Murphy will get the chance to add to his 50 goals for the club when he plays against Clydesdale Bank Premier League leaders Celtic on Wednesday.
The 23-year-old, who has made 214 appearances, is going out on a high by scoring five goals in his last four games and nine in 13, mostly from a wide left or attacking midfield position.
And McCall believes the skilful and pacey player can continue to improve.
McCall said: "That's the thing that has been pointed at Jamie, his consistency levels. But for a young lad, playing in the position he does, up front and wide left, you sometimes get that. But recently, not only his goals, his performances have been excellent.
"Potential-wise, can he go on and play for Scotland? I don't see why not, he has got all the tools."
McCall added: "I believe Sheffield United are the club he is possibly speaking to. Who knows, the way he has been playing in the last few months there could be other offers."
One player definitely leaving Fir Park is defender Jonathan Page. Motherwell have announced they have "agreed to release" the 22-year-old six months before his deal was due to expire.
Page, who signed from Portsmouth, made 27 appearances for Motherwell including several in European football, but he was loaned to Hamilton in September and is likely to remain at new Douglas Park for the rest of the season.