Sunderland won't sanction Lamine Kone's £18million move to Everton until a replacement defender has been lined up, the Echo understands.
Sunderland's best defender could have played his final game for the Wearsiders though, with cash-rich Everton moving quickly to replace John Stones.
Stones' £50million move to Manchester City was rubber-stamped on Tuesday morning, which then triggered movement in the transfer market.
Everton want to replace the England international with Kone AND Swansea City and Wales captain Ashley Williams, who travelled to Merseyside for his medical.
Once Williams' £12million deal is confirmed, Everton are expected to push for Kone on a five-year deal.
Chelsea are also tracking Kone, with manager Antonio Conte a fan. Their main defensive target though, remains Napoli centre back Kalidou Koulibaly.
Sunderland won't sanction Kone's sale until a replacement for the 27-year-old is secured, with the new season now less than four days away.
Everton captain Phil Jagielka has been touted as a possible replacement, going in the opposite direction, but the Toffees are not thought to be keen on that arrangement.
The Sunderland players had a day off from training at the Academy of Light on Tuesday, with reports Kone had travelled to Merseyside for talks on Monday night.
Kone is expected back at Sunderland training on Wednesday.
Sunderland - who have also been linked with a move for Torino right-back Davide Zappacosta - are not commenting on the transfer speculation.
Both Everton and Leicester have enquired about Kone this summer, with the Ivory Coast international understandably attracting interest after starring since arriving for £5million in January.
He is the club's biggest asset and any money from his sale would be ploughed back into adding much-needed new faces to the Sunderland squad.
There had been no indication during the course of pre-season that Kone wanted to leave Wearside, with the defender understood to have been wanting a new deal.
But Everton have ramped up their interest in the past few days and Sunderland could now cash in.
David Moyes, who signed defender Papy Djilobodji from Chelsea for £8million, is looking to add to his squad ahead of Manchester City on Saturday.
Moyes, though, has not ruled out banking big bucks from one of Sunderland’s key players in future, if that money can be used to strengthen the side.
Speaking last week, Moyes told the Echo: "I’m always going to keep the best players, but I’m also going to move the club along. At different times things will have to change.
"We had to sell Wayne Rooney, Mikel Arteta, Jack Rodwell, Joleon Lescott, but it didn’t affect the club, the club moved on.
"I would never say that it won’t happen. But I want to keep the best players and build a team.
"The more important thing is us bringing players in and bringing in players who can get us away from the wrong end of the league."
Ironically, Kone is the cover star and features in the latest edition of Legion of Light, the club's magazine.
Speaking in the magazine, Kone said: "Last year was a difficult end of the season but we managed to stay up so we 're very much looking forward to the new season, when hopefully we'll have a much better year.
"You have seen a lot of me so far but there is a lot more to come!"