The 18-year-old had a promising debut for the Reds cut short in the Carabao Cup third-round tie after 81 minutes having landed awkwardly following a challenge on the touchline.
Leoni will now be examined by the club’s medical team to determine the severity of the issue.
“Of course, he is down because for him it didn’t feel good immediately, but this is something now we have to assess,” Slot told his post-match press conference at Anfield.
“Normally these things don’t happen in five to 10 minutes and you have to wait for tomorrow to see how he comes in and then maybe do an MRI scan to know more about how serious it is.
“Normally the emotions of a player tell you a lot. I saw during the weekend a player in the Dutch Eredivisie who went out completely in tears – Ruben van Bommel, by the way, of PSV – and a day later it proved to be that it was right.
“Let’s hope for the best.”