The former Ibrox winger – who is now player/manager at Queen of the South – believes that gruelling games like their 2-2 draw at Peterhead on Saturday will provide a harsh reality check.
With Light Blues boss Ally McCoist's new ensemble of former SPL stars and Murray Park youth products facing a host of unfamiliar surroundings, Johnston has no doubt that the gelling process will need to be achieved quickly if that stark shock is to be overcome.
However, the 38-year-old has backed the superior quality being assembled by Coisty at Ibrox to overcome the challenges posed by life in the lower reaches.
He said: "With the level of change that has gone on at Rangers over the summer – and obviously there will be more comings and goings before their September 1 transfer deadline – you have to expect that early on there will be some turbulence.
"Even top quality players like Ian Black, Francisco Sandaza and Dean Shiels will take time to gel. You also have promising youngsters like Barrie McKay in there and the Third Division is going to be a culture shock for all.
"So probably early on, certainly away from home with the other teams in the Third Division fresh right now, Rangers can expect teams to have a real go.
"There is no doubt about it that Peterhead is not the only place Rangers are going to find tough on their travels.
"But once Ally has his squad assembled and his starting 11 nailed down and everyone knows what they are doing, then I come back to the bottom line that quality will come out on top for Rangers."
The Ibrox club have already signed Black, Shiels and Sandaza on long-term deals and more new faces are expected. Johnston has nodoubt that the stature of the Ibrox club has played a big part in players' decisions to play in the lower leagues.
He added: "I think that guys like Black, Shiels and Sandaza have signed for Rangers because they are a huge club with great tradition and that is always going to be the case.
"Obviously they have all signed long-term deals and I don't see them waking up in the winter and questioning if they got that right.
"You can't forget the fact that every other game will be at Ibrox in front of massive crowds and the opportunity to play there is something that a pro would relish.
"You can't underestimate how much pride a player takes on pulling on a Rangers jersey."
Meantime, Brazilian midfielder Andre Moritz has started a trial with Rangers.
The 26-year-old is available on a free transfer having left Turkish club Mersin Idmanyurdu in the summer. He has spent the last five seasons in Turkey, including spells at Kasimpasa, Fluminese, Kayserispor and Mersin.
And 21-year-old French defender Sebastian Faure is also training with the squad.