Hardie, who is 18 years of age and plays as a striker, joined Raith Rovers on an emergency loan deal earlier this month.
The youngster has moved to Ray McKinnon’s side for 28 days to help in their quest to finish in the Scottish Championship playoff spots.
The teenager was able to make just four first-team appearances for Rangers this season.
Hardie was unlikely to get much playing time for the Gers in the coming weeks, and Johnstone, who played for Rangers in the 1970s and 1980s and is a club legend, believes that it was a good decision of Warburton to send the youngster out on loan.
“Being on the bench and getting five or ten minutes here and there is no use for a young player,” Johnstone wrote in The Glasgow Evening Times. “You need to be playing games and playing at a good level.
“It is wonderful being involved with the first team at Rangers, but Ryan will know himself that this move is the best move for him to go to Raith.
“He is playing in a good side against the same teams that Rangers are up against every week and hopefully he will go there and get a regular game.
“It is a chance for him to go out, learn the game, work on his game and score goals. You have to be playing in competitive games to improve.”
Rangers are at the top of the Scottish Championship table at the moment with 60 points from 24 matches.
The Glasgow giants are eight points clear of second-placed Hibernian, who have a game in hand.
Meanwhile, Raith Rovers currently find themselves fourth in the standings with 32 points from 23 games.