Waghorn joined Rangers from Wigan Athletic in the summer of 2015 and has been a massive success at Ibrox.
The 25-year-old has established himself as the best striker in the Scottish Championship and has scored as many as 25 goals for the Gers.
Johnstone, who played for Rangers back in the 1970s and 1980s, rates Waghorn highly.
The former Scotland international has also backed the former Sunderland and Leicester City striker to score at least 40 goals this season.
“The one thing that I really like is his attitude,” Johnstone has written in The Glasgow Evening Times about Waghorn. “He works his socks off. He is a team player first and foremost.
“If he can square the ball for a tap-in, he will do that. Some strikers would always say ‘I am a striker, I am shooting and I don’t care who is in the middle’. They are blinkered at times.
“But Martyn is different in that respect and he is a really hard-working, industrious forward and the kind of player that Mark Warburton likes.
“I scored 41 goals in 1978, which included two for Scotland, and that was my best total for a season. I think he can beat that without batting an eyelid.”
Rangers are at the top of the Scottish Championship table at the moment with 47 points from 19 matches.
Mark Warburton’s side are three points clear of second-placed Falkirk and have a game in hand.
The Gers will return to league action on Saturday afternoon when they take on Livingston at Ibrox in Glasgow.
Rangers will head into the game on the back of a 5-1 victory over Cowdenbeath in the fourth round of the Scottish Cup at home.