The 18-year-old saw his Reds debut cut short on Tuesday evening at Anfield during a 2-1 Carabao Cup victory over Southampton.
Leoni landed awkwardly following a challenge on the touchline in minute 81 of the contest and required the help of medical staff in leaving the field of play.
Slot reported immediately post-match in midweek that he would require further tests to gain a full diagnosis of the problem.
Speaking ahead of Saturday’s Premier League trip to face Crystal Palace (3pm BST kick-off), the boss has now given further insight into the injury and was also asked if Federico Chiesa would now be expected to join his Champions League squad as a result.
“Yeah, the last thing is true,” Slot told reporters on Friday morning at the AXA Training Centre. “And the first thing is he [Leoni] is not in a good place, of course, because he tore his ACL and it means he will be out for a year.
“Being so young and coming to a new country and playing so well in your first game… it’s very hard to take the positive side. There’s never a positive side – but you always try to look at a positive side and that is that he is still so young, so he has so many years still to go after he recovers from a terrible injury like that.”