Phil Brown's side are currently in fifth place but Duffen is well aware they now need to clinch promotion in order to keep the 20-year-old Campbell at the KC Stadium.
"I have said consistently that the challenge is first Manchester United have to confirm they don't want him and then we have to be a Premier League club if we want to attract him," said Duffen.
"I don't see Fraizer signing for a Championship club. In fact, I would advise him not to, I think he's too good."
He told the Hull Daily Mail: "He's a great player and I'm sure he's got a great career ahead of him."