The former Queen of the South manager has had a difficult few weeks since replacing Kenny Shiels at Rugby Park earlier this summer.
Killie have won none of their four Scottish Premiership games to date and the 2012 Scottish League Cup winners were knocked out of this year's competition at the first hurdle after losing to Championship side Hamilton on Tuesday.
That sparked an angry reaction from supporters, with several already questioning 39-year-old Johnston's appointment.
But McCall, who welcomes Johnston's side to Fir Park for Saturday's early evening kick-off, believes the new boss needs time to find his feet.
He said: "I don't know who is giving Allan stick but that's not surprising nowadays because football is a cut-throat business.
"But results can change so quickly. You can lose a couple of games and then the next minute you win two on the bounce. It's far too early to be getting on managers' backs.
"Certainly in Allan's case, he has got a tough case and you will see the best of Killie once they get their own players in and playing the way he wants to play, because they have had a big turn around.
"As a manager, you accept that when you have had a couple of blips people are quick to get on your back. But it's far too early to be giving people criticism."
The one bright spark for the Killie support has been the form of Nigerian playmaker Rabiu Ibrahim.
But the 22-year-old says even he must redouble his efforts to help the team out of their current slump.
He said: "We can't figure out why this has happened. I don't know what the problem is. We are all sad. Nobody is happy. But we are strong. We can recover from this. Maybe we just need to work harder.
"I have to work harder myself. I have to muck in. I can't just keep going forward and not come back and help out.
"I know I'm not too good at mucking in right now but I'm working on that."