There was hope the Germany attacker would be fit for Saturday's clash with West Ham at Anfield, but Slot says Wirtz's recovery is still ongoing.
However, there is better news for wing-back Jeremie Frimpong, with Slot declaring the Dutchman will be in the squad for tomorrow.
He said: "Jeremie is OK, trained with us and is available, don't expect Florian to be, which is a disappointment, every time my player isn't available is disappointment and I didn't expect. No (idea how long he is out) usually I do but don't want to share it but I honestly don't know.
"It's a big accomplishment for him to stay fit for such a long time because he had to stay fit to make the step up but unfortunately he is not available, which is a disappointment for him and for us, because he played really well."
Hosting West Ham
Liverpool will host the Hammers, sitting in sixth place on the Premier League table, three points outside the top four.
Slot continued: "The way I see it, this is another game for us and we are not focused on other teams, only ourselves. We need to win a lot of games, because our teams will and that is where our focus is. Not so much about points and gaps although we are aware of the table."
Given his team's inconsistency this season, particularly against teams in the bottom half of the table, Slot accepts West Ham represent a genuine threat given the Reds' form.
He conceded: "Data tells you sometimes something that is right and can be manipulated but I cannot say we haven't struggled against the promoted sides, so we understand at this moment how difficult winning games is, regardless of position.
"But that is not only for us. You watch games of the opposition and we are not the only team that sometimes plays a half like we played against Forest. It's also the quality you face, give credit to Forest. That is the new reality in PL, in recent years the top teams, it was more obvious. Now it is about all the money in the league and these teams have owners who can spend.
"The win in November against West Ham was an important win, we played well. Resulted in a win, only lost twice since then, only in extra time, which is nothing new for us this season! Maybe things have changed, finally, as we scored stoppage time v Forest."
Salah and Alisson
Slot also addressed the situations of two of his senior veterans.
Brazil goalkeeper Alisson has been linked with a move to Juventus this week, with the Bianconeri readying an offer for the summer.
Slot said of his senior keeper: "In an ideal world you don't need the goalkeeper because you defend so well but the good thing is if we do we have an outstanding one, he has been here seven years now, we don't only have one good goalkeeper, we have (Giorgi) Mamardashvili too, another who could be No. 1.
"This place is in a good place for keepers, has been for years. Ali always been No. 1, which says how good he is."
Meanwhile, Mohamed Salah has again drawn criticism from pundits for his form in recent weeks. Many in the media have been calling for Slot to bench the Egyptian, who is also being linked with a summer move away.
The Dutchman stated: "He set his own standards and they are so, so, so high the moment he doesn't score for a few games, people are surprised, so that is the big compliment.
"I think I should go really detailed into performances and this is not the time and place to do it here. With Mo we are used to scoring goals. We also know, even in my time, it has happened, I've had questions about Mo not scoring for three or four games but he always scores again."
Bringing through Ngumoha
Looking to the future, Slot was asked about how he has managed the minutes of teenage winger Rio Ngumoha. The Reds manager says he is giving the 17-year-old more chances as his game develops.
He explained, "Yeah, another great example how you can manipulate data, not talking about cup and Champions League games, Rio has only played 89 mins PL, to be more realistic - or to tell the truth! - then you see his playing time has improved massively over the last few weeks and that is because he has developed.
"People are talking about him because he is doing really well, we like that. It's another example of a player that someone want to see more and more. It's all due to the fact he does well when he comes in. The highlight versus Forest was his 1v1 action and how important that is in football but he had a few other moments he did as well which we talk about.
"That is part of the development, I really like where he is now. If he stays fit more minutes will come in the future."
Slow starts
Meanwhile, Slot also conceded this morning that they're yet to work out how to find a way through teams like West Ham in the opening minutes of games at Anfield.
Liverpool have been consistently poor starters at home this season and Slot concluded: "It's never down to our approach, we've seen many times at Anfield...when I had to play at Anfield, I would 100% ask my players to slow the game down and get the fans not behind the team.
"Take your time, slow the fans down and that is what other teams do, which is fair. So it is on us to find the moments to create. It's harder to do when the other team is totally fit."
