David Moyes has suggested Jordan Pickford could be worth as much as £30 million, with the Sunderland manager rejecting "derisory" valuations of the highly rated goalkeeper.
Pickford has been one of the few bright sparks in a dire campaign, with Sunderland relegated to the Championship after 10 years in the Premier League.
The 23-year-old has been linked with a raft of clubs, including Liverpool and Everton, although Arsene Wenger ruled out a bid to bring him to Arsenal.
Pickford was called up to the senior England squad for the first time in October and Moyes indicated Sunderland will be looking for a substantial fee for their prize asset.
"It would need to be a really, really big offer," said Moyes.
"I have said all through the season that sometimes on your journey, you may have to sell to improve.
"But I have heard some really derisory sort of figures getting mentioned and it wouldn't be any of those figures, I can tell you that."
Only one goalkeeper has ever moved for a fee in excess of £20million, with Gianluigi Buffon's £33million transfer from Parma to Juventus making him the world's most expensive stopper.
But, asked by reporters if Pickford's fee could rise as high as £30million, Moyes said: "Yes. He has been our highlight.
"For a young boy to break in and perform like that. The other night he was fantastic against Arsenal.
"Nearly everything was perfect and we will need him again this weekend at Chelsea."