Wirtz is yet to score for Liverpool since making a £116M transfer from Bayer Leverkusen in a move that has already been named as one of the worst in the club’s history.
The German international picked up an injury during November's international break and was missing for the 3-0 loss to Nottingham Forest on Saturday afternoon.
Now, as reported by the Liverpool Echo, Wirtz was not involved in first team training as the club prepare for Wednesday night’s game against PSV at Anfield.
Wirtz, along with Conor Bradley and Jeremie Frimpong were all missing from Slot’s training selection which only raises more questions about the Reds’ defence and midfielder which has looked incredibly weak this season.
Rio Ngumoha, Trey Nyoni and Kaide Gordon all joined the session whilst Calvin Ramsay, who could replace Frimpong or Ramsay also attended but is ineligible for the Champions League opening stage.
Slot’s side have taken nine points from four games at the halfway point of the Champions League opening stage and could climb into the top four with a win against PSV, who have collected five points after winning just once so far.