The 25-year-old central defender ended months of speculation by turning down offers from the English Championship to commit his future beyond this summer to Rangers, with the suggestion that the Scotland internationalist has signed a four-year contract at Ibrox.
Rangers will now try to prise Souttar away from Hearts now rather than wait until June, although the Jambos say that they will only sell for a substantial bid.
And Stewart reckons that Hearts are unlikely to sell given his importance to the team, with a top-three finish on the cards.
"Hearts are a better side with John Souttar in the team, so I don't see there being any issue at all [with the playing him],” Stewart said on BBC Radio Scotland.
"Robbie [Neilson, Hearts manager] has to protect him, I don't think there will be any serious backlash and he des play him, of course he does.
"But this if football and deals get done all the time
"I'd be very surprised if this deal gets done in January personally.”
Ex-Hearts defender Allan Preston also questioned whether Souttar would move west this month.
"It you are talking about half a million quid that Hearts are looking for and rightly so that's a lot of money for Rangers to spend for a player they could be getting for free in six months time,” Preston said.
"So I would be very surprised if a deal is done in this window but football is full of surprises."