The Reds lost their 43-goal forward to a suspected dislocated shoulder in the first half of their Champions League final defeat in Kiev.
Salah was in tears as he left the NSC Olimpiyskiy Stadium pitch and, with just 20 days to go until Egypt face Uruguay in their tournament opener, Klopp concedes that the 25-year-old's participation is in major doubt.
"Yes, unbelievable," he told BT Sport when asked if Salah could miss the World Cup. "It is very serious. It is with his shoulder. He would have tried to play on [if he could have] but I have not seen him. He is possibly in hospital for an X-ray. It does not look good."
Salah ended the season three goals short of Ian Rush's long-standing record of 47 in a single campaign for Liverpool.