The shot-stopper, signed for £6million from Manchester United in 2018, is set to make his 100th appearance for the Baggies at Brighton on Monday.
Johnstone enters the game in impressive form having made some excellent saves in the recent clashes against Southampton and Burnley.
Bilic believes in Johnstone, David Button and Jonathan Bond, he has three very good goalkeepers in his squad.
But the boss admits he has been delighted with Johnstone’s recent displays.
“Without a good keeper, nobody has got a chance,” Bilic said.
“We have good goalkeepers but I am extremely happy with him (Johnstone).
“At the beginning, because this is his first season playing in the Premier League, it was a new challenge for him.
“You would read it’s Sam’s first time in the Premier League how is he going to do?
“They (the players) read it all, it all goes in their head.
“Then after the Everton and Chelsea games there were a few comments – could he have saved this one or that one.
“Then he had a couple of good games, a few good saves against Southampton.
“A couple of good saves against Burnley and he is confident.
“And that is what we need. We need that kind of performance week-in, week-out if we are going to stay up.
“Hopefully he is going to give it. He is in a good way. He is very confident now.
“We are going to do everything so that he is challenged by our other goalkeepers.
“But we want him to be confident, to maintain this level and even improve.”
Meanwhile, Bilic says new recruit Karlan Grant has quickly settled into life at The Hawthorns.
The striker was training on his own with Huddersfield before joining the Baggies just a day before the transfer window closed. He then made his Albion debut three days later against Burnley.
“Karlan didn’t have time to settle in but he didn’t need time because he is a guy from England,” Bilic said.
“He knows a lot of the players, he played in youth academies with some of them, he played in the same league – whether League One or the Championship.
“He didn’t need to introduce himself.
“That is the reason why it was possible for us to start him against Burnley after only three sessions with us.
“He has had a bit more time to gel and he looks good in training.
“He is fit, he is a very lean player but we still have to get him match fit. What he needs is match fitness, routine and a bit of knowing the movements of the other players on the pitch. He can get that through training but the best way is to get it through matches, and that’s the same with match fitness.”