McCall confirmed Murphy was due to be holding discussions with another club 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.
"Potential-wise, can he go on and play for Scotland? I don't see why not, he has got all the tools," he said. "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 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."