The Glasgow giants will take on Scottish Premiership side Dundee at home in the quarter-finals of the Scottish Cup on Saturday afternoon.
Johnstone, who played for the Gers in the 1970s and 1980s and is a club legend, believes that Mark Warburton’s side will clinch the tie at Ibrox in Glasgow.
The former Scotland international, though, has warned that Rangers will have to defend well in order to win the game and progress to the semi-finals of the Scottish Cup.
“I think it will be a very open encounter and the Rangers back four have really got to be on their game,” Johnstone wrote in The Glasgow Evening Times.
“They will come up against better strikers than they have faced this season and if they make any mistakes then they will be punished.”
“It could be a really good, open game,” added Johnstone. “Rangers will have to defend really well and take our chances at the other end. There is no reason why we can’t win and get into the last four, though.”
Rangers are going strong in the Scottish Championship at the moment and will head into the game against Dundee high on confidence.
The Gers got the better of Raith Rovers 2-0 at Ibrox in the league on Tuesday evening.
Dundee will be a tough opposition, especially as they are doing quite well in the Scottish Premiership and played out a goalless draw with Celtic at Celtic Park on Wednesday evening.