Rangers appear to have slowed down in the last few weeks.
The former Rangers star believes that Gers manager Mark Warburton needs to sign a big striker and a stronger, holding midfielder.
“I still think they need a big striker,” Johnstone wrote in The Glasgow Evening Times. “I know Ryan Hardie is there and he will definitely get his chance soon but they might want a ready-built one to come in to lend some more support to Martyn Waghorn who has often looked best cutting in from the wide areas.
“If Foderingham has to kick to long then it would useful to have someone up there with a chance of winning the headers and linking up with others.
“I also think they need a stronger, holding player in midfield that they maybe don’t have at this time. They’ve got various midfielders who want to get forward but on the break how many goals have Rangers lost when they’ve overcommitted to attack?”
Rangers are at the top of the Scottish Championship table at the moment with 37 points from 14 matches.
The Glasgow outfit are ahead of second-place Hibernian only on goal difference, although Hibs have played a game more.
Warburton’s side appear to have slowed down in the last few weeks, and Hibernian seem to be launching a genuine challenge for the Scottish Championship title.
Rangers suffered a 2-1 defeat to Hibs away from home at Easter Road at the start of the month and were held to a 1-1 draw by Livingston at the Tony Macaroni Arena this past weekend.
The Scottish giants will take on St. Mirren at Ibrox in Glasgow in the Scottish Challenge Cup semi-finals on Saturday afternoon.