The Scotland cap is a big loss but Neilson is taking a two-pronged approach as he looks to develop Toby Sibbick and capture some signings.
Hearts manager Robbie Nielsen will recruit to replace John Souter in the summer.
But he hopes Toby Cibic, who signed in January, can eventually rise to the level of a Rangers-bound defender.
In January it was announced that Sauter would be stepping down from M8 in the summer to join the Premiership champions.
A deadline was not dealt for this season and although there were some boos to start from the stands, Jambos remained largely behind Souter with support, as he continued to perform.
But Nielsen knows it can't ignore Number Four's impending departure - and so the plans are in place now.
In January it was announced that Sauter would be stepping down from M8 in the summer to join the Premiership champions.
A deadline was not dealt for this season and although there were some boos to start from the stands, Jambos remained largely behind Souter with support, as he continued to perform.
But Nielsen knows it can't ignore Number Four's impending departure - and so the plans are in place now.
"The plan was to get him in the January window and work with him until the summer. Then we'll try to recruit then too.
"Leaving John leaves a big hole we need to fill."
Souttar has remained professional since the announcement of its pre-contract with Ibrox.
There hasn't been a drop in performance, but it's clear he's helped when center-back sidekick Craig Halkett is with him.