The Canadian international's future is likely to remain a talking point if the Premier League side decide to step up their pursuit before the transfer window closes.
Reports suggest there remains a significant gap between Everton's valuation of the right-back and the figure Celtic would demand to sanction his departure.
With Johnston still under a lengthy contract at the Hoops, Stewart believes the Scottish champions are in a strong negotiating position and have no reason to accept a cut-price deal.
Speaking on Premier Sports, Stewart said: "I was going to say was the other thing in terms of the value, obviously his contract situation, and I don't know what they've just said there. It depends how long he's got left in his contract. If he was sitting with two years left on his contract, then for me, you're looking at at least £20 million for him.
"You know he's been at the World Cup, starring for his country. He's playing in the Champions League and in Europe, excelling.
"I just think that he's got three years left in his contract at this point. Just for argument's sake here, if they don't get circa £20 million and above, Celtic are in a position where they say, "Well, we don't want to sell", and then they'd probably say Alistair Johnson, "Okay, one more season. We'll go at two years left in your contract, and you're still retaining top value because two years in your contract, you can still look to sell at that point.
"I don't think there's any need for Celtic to be looking to sell him for anything less than £20 million."
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