The 25-year-old former Sunderland and Wigan Athletic man failed to convert a spot kick against Falkirk away from home in the Scottish Championship on December 19.
Rangers lost that game 2-1 at the Falkirk Stadium, and there are now suggestions that they could slip up in the race to win the Championship title this season.
Club legend Johnstone, who played for the Gers in the 1970s and 1980s, believes that Waghorn should continue with his penalty duties for Mark Warburton’s side.
“Martyn has missed two now but I think he will want to take the next one,” the former Scotland international has written in The Glasgow Evening Times.
“If he is playing well and he believes in his own ability to score, then he will step up. and hopefully put it away. He is still the No.1 penalty taker until he says he doesn’t want to take it.”
Rangers will return to action on Monday afternoon when they take on Hibernian at home in the Scottish Championship.
The Gers are at the top of the league table at the moment with 41 points from 17 matches. Hibernian are level on points with the Glasgow giants, but they are second due to an inferior goal difference.