Wirtz, a summer arrival from Bayer Leverkusen, will make his Premier League debut tonight against Bournemouth.
Slot says handling the physical demands of the Premier League will be Wirtz's first major test.
“Everything we expect is very positive but between expecting and realising something is different,” said Slot. “Why is Messi and Ronaldo and Mo, why are they so highly regarded? Because they are always fit.
“Every single game they are fit and that is something that Florian first has to show in a league like ours. If he is able to stay to fit then our fans will see great moments.”
Slot added, “Factually, we can see him as a young player but in terms of his experience, I don’t see him as a young player.
“He won the league with Germany, won the cup, has played for his country numerous times and apart from that, his mentality is his biggest strength.
“That says a lot because if you see him play, you’re like: ‘Are you serious because he’s so creative’. But how mentally strong he is, he doesn’t get distracted at all by a transfer fee or anything else.”