The 27-year-old glovesman has been in top form for the Baggies ever since they returned to English football’s top tier.
And his heroics continued against former side Manchester United on Saturday with Johnstone making a series of outstanding stops to deny the likes of Anthony Martial, Marcus Rashford and Harry Maguire.
Data specialists Opta each week produce statistics they refer to as ‘post-shot expected goals’.
It means the number of goals a goalkeeper should concede based on the direction of shots they face and their position at the time.
But according to those stats, Johnstone should have conceded 21 goals instead of the 17 he has.
Villa’s Emiliano Martinez is the second best performer with Chelsea’s Edouard Mendy third.
Prior to his heroics at Old Trafford, Johnstone was disappointed with his role in Harry Kane’s 88th-minute goal in the 1-0 loss to Tottenham.
Bilic, though, is overjoyed with how well his goalkeeper has adapted to life in the Premier League.
“I said to myself, my staff and the guys, five things that we needed to be constant at the top level to have a chance of staying up and one of them is the goalkeeper,” Bilic said.
“Sam is doing his part of that puzzle.
“He’s been excellent for us. Against Spurs, OK, he was a bit unlucky or he made a wrong decision.
“But he also made one great save against Spurs.
“Sam is in a good zone and I didn’t doubt it.
“He’s found good confidence as a new goalkeeper in the Premier League.
“He’s growing and is excellent at the moment and that’s what we’re going to need for the whole season.”
Meanwhile, Sheffield United’s Chris Wilder – who face Albion in a crucial clash on Saturday – has dismissed claims he should be worried about his job.
The 53-year-old has done an outstanding job at Bramall Lane guiding the Blades from League One all the way to the Premier League.
But his side are without a win in 12 top-flight outings, a run which dates back to last season.
“It’s great how you guys talk about people’s careers so flippantly, I love all that,” Wilder said when asked about his future.
“I haven’t had the sack in 20 years – 911 games if you hadn’t noticed – I don’t fear it.
“I need to be careful because I don’t want to come across as arrogant and look like I think I’m untouchable, but I don’t feel it should be asked given the journey that we have been on over the last four years.”
Wilder’s words prompted George Baldock to come out in defence of his manager – with the defender saying he believes United are close to a first league win of the campaign.
“What this manager has achieved is absolutely magnificent,” he said. “He has not got here by luck. We have got the perfect man in charge to put things right.
“The boys will run through brick walls for him.
“We are not that far away. The manager speaks about it a lot, if we punish teams a little bit more we will be right in games.”