The 18-year-old, who joined from Parma this summer, was stretchered off the pitch at Anfield in the 81st minute during Liverpool’s 2-1 victory over the Saints in the Carabao Cup.
Before the injury, he had delivered an impressive debut performance, which Reds manager Arne Slot described as "great" in his post-match interview.
Leoni's injury concern
Speaking about Leoni's injury, the Dutch coach said, as reported by Daily Mail, "He's down because for him it didn't feel good immediately."
"But this is something now we have to assess, and normally these things don't happen in five to 10 minutes. You have to wait until tomorrow to see how he comes in and then maybe do an MRI scan to know more.
“Normally, the emotions of a player tell you a lot. I saw a player in the Dutch Eredivisie at the weekend who went out completely in tears and it proved to be that he was right [that he had suffered a bad injury], but let's hope for the best."
While Liverpool has not made an official comment on Leoni's injury, it is understood to be an ACL tear.
The club is believed to have been waiting for the swelling to subside before a definitive diagnosis could be made.
Liverpool face a difficult trip to face Crystal Palace on Saturday afternoon, where they will be without both Ekitike and Leoni.