Johnstone will steer the champions against Benni McCarthy's men at Bidvest Stadium in the absence of head coach Gavin Hunt.
Hunt was handed a two-game suspension after being sent off in the club's MTN8 quarter-final win over Golden Arrows at the same venue last week.
Johnstone appears calm regarding his temporary elevation to the hotseat.
"We work in tandem and sing off the same hymn sheet, so we know what we are doing week to week, the expectancy of the players for pre-match, half-time and whatnot," said Johnstone when asked about taking over the duties from Hunt.
"It's not like Gavin is not gonna be around, he will be there. I work under Gavin and it's important that I have got his back. It's not an opportunity for me to stand up and be counted and make myself known. I'm here humbly, I don't need to be known. I'm carrying out the instructions based on what we do day in and day out. It's not about me, it's about the team."
The Clever Boys recorded their maiden topflight triumph in 2016/17 after coming second and third in the previous seasons, with Johnstone eager to build on that success.
"I think where we have come short in previous seasons is not having the actual belief that we can go out there and win the title and we proved it last season, that we can and we did. Winning the MTN8 and the league is huge for the club. Yes it goes with more pressure, now we have to go from strength to strength and try and do it again. There is certainly a lot more belief around the club and we have to go out there and do it – we have to, there is no other option."
Johnstone was initially hired as a fitness coach by Wits but took up a technical role with the departure of head coach Roger de Sa in 2012. Johnstone, who ended a nine-year PSL playing career under De Sa, says he has learned much from his former bosses, as well as incumbent Hunt.
"As a player, you pick up things every day from your coaches, some things you take with you that are massively beneficial while others you discard but you are always learning. I always felt that every day was a learning opportunity. Even now under Gavin Hunt, every day is an opportunity to learn."