The 24-year-old has struggled to play regularly for the Rangers first team so far this campaign.
The former Queen of the South man has been on the books of the Gers since 2013, and former Rangers star Johnstone believes that the striker needs to impress manager Mark Warburton whenever he gets some playing time.
“Life at Rangers is proving difficult for Nicky Clark and that is unlikely to change as the club seeks to bring in new players in preparation for a return to the top flight,” Johnstone wrote in The Glasgow Evening Times.
“That is why he must continue to do what he did against Livingston in the quarter-final of the Petrofac Training Cup and make the most of chances that come his way.”
Johnstone was referring to Clark’s performance against Livingston in the quarter-finals of the Scottish Challenge Cup at Ibrox on Tuesday evening.
Clark came on as a substitute after the break in place of Tottenham Hotspur-owned winger Nathan Oduwa and scored what proved to be the winning goal in the 75th minute with a low effort from 30 yards.
“He only has a short space of time, though, to convince Mark Warburton that he can be a valuable player in the longer term,” added Johnstone.
“If Rangers do go up, there will be reinforcements in the summer. There may even be a need for a taller striker to come in right now.
"Clark, though, is the type who can come on and change a game. His future is in his own hands. He has to score goals every time he plays.”
Rangers have been the best team in the Scottish Championship so far this season and are on course to winning automatic promotion to the Scottish Premiership.
The Glasgow outfit have picked up 30 points from 10 matches in the league and are as many as eight points clear of second-place Hibernian.
Rangers will take on St. Mirren away from home at St. Mirren Park in Paisley in the Championship on Sunday afternoon.