McCall confirmed Murphy was due to be holding discussions with another club today 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 SPL 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.
"First and foremost he is a great lad. He just wants to learn and get better. He's a good listener but more than anything he has really top-class ability.
"He can go both ways, he can play wide, play behind the striker. He can finish with both feet and his head. It's just that little bit of consistency he has probably lacked.
"But he has been a great player, I have been very lucky to have him here. He can go on to better things, that's for sure."
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."
McCall was given the task of leading Motherwell into the top six of the SPL and selling one player each season when he took over two years ago this week.
After selling Mark Reynolds to Sheffield Wednesday in his first few weeks, Motherwell did not bring in a transfer fee last season, but went on to finish third.
The fee for Murphy is likely to be around the £100,000 they received for Reynolds, although there could be extra money to follow, but McCall has been told he does not need to sell anyone else in January.
"I think I've turned Jamie from a million-pound player into whatever the fee is going to be, so I've not been doing my job really," he said.
"But from a selfish point of view, I've been delighted he has been here so long.
"Jamie has helped us get to third and I feel as though I've done part of my job that we've managed to sell someone.
"With his contract being up in the summer, we are not getting fortunes for him. We are not getting anywhere near what he's worth.
"If it's Sheffield United or anywhere else, they are getting an absolute bargain. But our hands are tied with the length of contract."
Lech Poznan have been linked with a move for Henrik Ojamaa but McCall revealed it would take a much higher offer than the £170,000 quoted to prise away the Estonia striker.
"I'm happy, as much as you can be, to lose a player," McCall said.
"Nobody can say never but unless something comes that the club cannot turn down, then it will be one player.
"And I think Henrik can only get better. He has only got two goals this season but if he adds goals to his performances, he is going to be worth a lot of money to this club and the money that's been offered has been nowhere near."