The 25-year-old will join Rangers in the summer when his deal at Tynecastle Park expires having agreed a pre-contract in January.
Souttar has been a key player for Hearts this season, making 30 appearances and building a formidable and balanced partnership with Craig Halkett, whether it has been as part of a four or three-man defence, with their attributes complementing one another.
Toby Sibbick was brought in on a three-and-a-half year deal during the January transfer market with a view to becoming an important player in the defence.
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Manager Robbie Neilson also revealed the club would be in the market for someone with similar qualities to Souttar.
“We brought in Toby Sibbick and we hope with development we will get him to that level,” he said.
"The plan was to get him in the January window and work with him until the summer. Then we will look to recruit then as well.
"John leaving gives us a massive hole we need to fill.”
From the summer, Hearts will be required to form a new central defensive partnership involving Halkett, who has arguably been the team’s most improved player this campaign.
Since his return to the team against St Mirren last week it has been two wins from two and two clean sheets.
“He’s been great," Neilson said.
“Him and John Souttar together give you that platform to go and play. They are physical and strong in the air, they dominate the striker and are also very good on the ball. Having the two of them is great.”