Liverpool head to Old Trafford three points behind third-placed Manchester United with four games remaining. Aston Villa are level with Liverpool on 58 points in fifth, meaning the race for the top four is tight enough that Sunday's result could reshape the picture significantly. A win for Slot's side would draw them level with United and potentially lift them above their rivals on goal difference — with direct Champions League qualification the prize.
Slot spoke at the AXA Training Centre on Friday morning and was emphatic when pressed on whether Wirtz would perform against elite opposition.
"I don't only hope that, I am 100% sure he will turn up. His improvement — he was already an elite player — has been for me obvious. Ninety-three minutes against Palace, wins a throw-in, and then hits it in the top corner. I was not convinced he could do that eight months ago. Every player is at his best at 25 or 26. He can only become better. That is the beauty of the signings we made."
The bigger concern ahead of the fixture is availability. Mohamed Salah will not feature after picking up a hamstring injury against Crystal Palace last week, though Slot confirmed the problem is minor and that the Egyptian is expected back before the end of the season. Goalkeeper Alisson Becker is also a doubt, having not yet returned to full training.
"Alisson hasn't trained with us yet. I will find out today if he can train today or at the start of next week. Very close but we don't take risks. There's a difference between a keeper and an outfield player — they need to run more."
On Salah, Slot was measured but relieved.
"We expect him back for the final part of the season. Not for tomorrow. It is always a big relief that his injury is minor — to play for us and for the World Cup. Any time a player who deserves a send-off gets the chance to have one, it's great news."
Alexander Isak is available after injury and Slot was candid about the need to rebuild his sharpness gradually.
"The more he touches the ball the bigger the chances. It makes sense that connections need to be rebuilt — that is the good thing, we have training time. A game is 95 minutes, full throttle, constant running, so being fit is quite important."
Curtis Jones, recently deployed at right-back, continued to draw praise.
"He has taken it on very well, not perfect — but who is perfect? He has hardly played it in his career but the way he does it is a big compliment. He was even better defensively against Palace than against Everton. Usually the ball is in safe hands with Curtis."
Milos Kerkez is expected back in training on Friday or Saturday after a minor niggle, while Dominik Szoboszlai is confirmed available.
Slot's assessment of United was sharp. He argued that Carrick's side are significantly better than their league position earlier in the season implied and that the transfer window has allowed their quality to finally show.
"Last season I said their players were better than the position suggested. Another transfer window added players and I would say that resulted in us finally seeing how good they are. They have become under Carrick more consistent in their results."
He also confirmed that Liverpool are actively looking to replace backroom coach Aaron Briggs, who departed earlier this season.
"We parted ways, so it makes sense we are looking. We look at where we can strengthen — it could be a player but also a staff member."
Despite the stakes, Slot declined to frame Sunday as anything other than the next game to be won.
"We are aware it is a big game — not only because it's United, but also for Champions League and the highest possible position. We have to prove ourselves every three days. It's up to others to judge, not me."
United have won their last three matches and sit eleven points clear of sixth-placed Brighton. Kick-off at Old Trafford is 3.30pm on Sunday.