Wirtz started in last night's win against Atletico Madrid, though had another quiet game.
However, Slot insisted afterwards: "He was indeed a few times close to scoring a goal and he was also a few times close to assisting a a goal – I think he was the one that created most chances for us today. So I can see with him also that he is getting fitter and fitter and adjusting more and more to us, which is normal.
"You need a bit of time to adjust. We all know how much quality he has but sometimes people forget that he is a 22-year-old that goes for the first time abroad. I am 47 now because it is my birthday today but last year I came when I was 46 and I know it was my first time working abroad, and in the first two or three months your whole life is changing.
"All the things that are normal to you are not normal anymore… because everything is different and that sometimes costs energy as well. Combine that with going to a new club, a new playing style because Leverkusen played 3-4-3 and we play 4-3-3 and so we ask a bit more from him off the ball and defensively as well.
"But I can see him growing, growing, growing more and more and more and I think that is what you can expect from a quality player, who always adjusts to what he needs. And he and the team will get better and better, but I liked his performance today already a lot."