Harkins, who is 31 years of age and plays as a midfielder for Dundee, recently taunted Rangers by claiming that the Ibrox outfit are a new club.
"They’re just a new club,” the former Kilmarnock and St Mirren midfielder told The Daily Record. “I’ve not played them before, so I’m looking forward to that. It should be good.”
Harkins was speaking ahead of the Scottish Cup quarter-final tie between Rangers and Dundee at Ibrox in Glasgow this weekend.
Johnstone, who played for Rangers in the 1970s and 1980s and is a club legend, believes that the Dundee midfielder made the comments as a joke.
“There has been a bit of a storm over Gary Harkins’ comments about Rangers but I don’t think the Ibrox crowd will bother too much when Dundee visit in the Scottish Cup next Saturday,” the former Scotland international wrote in The Glasgow Evening Times.
“When he said what he said, he was making a joke. It was stupid and he shouldn’t have said it, but I think it was a joke and he was smiling.”
Rangers are at the top of the Scottish Championship table at the moment with 66 points from 26 matches.
The Gers are as many as 11 points clear of Hibernian, who are currently second in the standings.
Mark Warburton’s side will return to action on Tuesday evening when they take on Raith Rovers at Ibrox in Glasgow in the Scottish Championship.