The centre-back is free to speak to clubs from December 1 when he enters the final six months of his contract at Tynecastle.
It has been reported that up to nine sides from the English Championship are keen on the 25-year-old, with Celtic and Rangers also linked.
Hearts boss Robbie Neilson is hopeful that Souttar will sign a contract extension to stay at the club, but has ruled out the player being sold on the cheap.
"John’s a key player for us,” he said. “We’re in negotiations with him just now. Obviously there is interest, because he has come back into really good form and fitness, he has played for Scotland, scored a goal.
“From our perspective he’s here until the end of the season, unless we get a big offer we can’t refuse. Hopefully within that time we can negotiate a new contract.
“The owner and [sporting director] Joe Savage have been speaking to him. We’ve put forward a really, really good offer and it’s now a case of trying to negotiate that.
"We’ll try as hard as we can to get him.
“If he does decide it is time for him to move on it will either take a huge bid in January for that to happen or he will see out his contract and hopefully we all leave happy.”
Neilson confirmed there has not been any contact with interested parties and is keen for the player to remain at Tynecastle until the end of the season.
“It’s a matter of the club, the player and the agent talking, negotiating, trying to get something done," he said.
“John for us is a key player. John will hopefully help us finish as high up as we can in the league. John will hopefully help us go as far as we can in the cup so you have to balance off the two sides of it. What that is worth and what a transfer is worth.
“From my perspective as a coach I will be pushing to keep him here as long as I can because he is such a key part of the team.”