Rangers have already won the Scottish Championship title this campaign and will play in the Scottish Premiership next season.
Celtic will have a new manager in charge of the first team during the 2015-16 campaign, as current boss Ronny Deila has stated that he will leave the Bhoys at the end of the season.
Johnstone, who played for the Glasgow giants in the 1970s and 1980s and is a club legend, believes that a managerial change at Celtic Park should not affect Ibrox side Rangers’ plans in the summer transfer window.
“In the words of the manager, with the greatest respect, it doesn't matter what Celtic do,” Johnstone wrote in The Glasgow Evening Times. “He has got a big task at Rangers to get us challenging for the Premiership as soon as possible and that is all he can look at. It doesn’t matter who Celtic appoint or what players they bring in.
“He can’t do anything about that. It doesn’t matter who the next manager is and we will have to wait and see who the Celtic board decide to go for.
“Mark will have his sights on a handful of players that will come in and help us challenge Celtic next season. It is the Rangers squad he has to look at, not the Celtic manager.”
Celtic are likely to sign players in the summer transfer window and will have a very strong squad next season.
Rangers are also set to add new faces to their current squad, but they will still not have a squad as strong as the Bhoys.
For the Gers, the 2016-17 campaign should be all about finishing second or third in the Scottish Premiership.