The Tynecastle Park club have rejected a bid understood to be around £300,000 from Rangers for the centre-back.
Hearts are believed to want upwards of £500,000 for the player this window.
Souttar has agreed a pre-contract to join the Scottish champions in the summer and was left out of the squad for the Scottish Cup tie with Auchinleck Talbot at the weekend due to injury.
While he remains a Hearts player he will be part of the squad building up to the Celtic game at Tynecastle this week.
"John will train today so I expect him to be fully fit for Wednesday and I don't see anything changing until then,” Neilson told Sky Sports.
"It's no problem at all, things will be bubbling away in the background. Whether another offer comes in or not I don't know, we'll wait and see.
"But at this moment John is a key player for us and he's giving 100 per cent every day in training. When he's been asked to play he's given 100 per cent as well, so I don't see that changing.
"I think all we have to do is just take it day by day. We've got a game on Wednesday, we'll prep for that and make sure John's part of the group for that.
"Then we'll see what today brings, what tomorrow brings."
Meanwhile, Neilson is still keen to add one or two players to his squad this month.
Hearts are going forward with a smaller squad but the club are looking to bring in attacking reinforcements with Saturday's match at Talbot highlighting the “need to add numbers”
"We're still working on trying to bring a couple in," Neilson said.
"As everyone knows, in this window it can be difficult to get the quality you want but we've got a couple of targets we'd hope to get done.
"We'll see where it goes. We don't have a huge squad, we made the decision at the start of the season to run with more quality than quantity and you saw at the weekend there we only really had three subs on the bench.”