Sadio Mane is facing a stint on the sidelines after trapping his knee in a challenge with Leighton Baines during the Merseyside Derby.
Liverpool striker Mane appeared in serious discomfort but was able to limp off with the help of medical staff.
Mane looked to have his leg bend into an unnatural position after tangling with England full-ball Baines.
Jurgen Klopp said:“Today we played a really good game. Nearly everything was perfect, we win it, the performance was good [but] then we lose Sadio Mane.
“When I saw him in the dressing room, it didn’t look like he’ll be ready for Wednesday.
“We will see, hopefully it’s not that serious, but of course it’s not nothing and that makes our life not easier, I would say.”
When asked to clarify what type of injury Mane had sustained, Klopp replied: “I don’t know exactly, I don’t want to speculate now.
“Obviously he has pain and it didn’t look like it’s only a knock or a muscle or something, but we have to wait.
“When I came in I was looking for him immediately and I saw him on the bed. He’d already had treatment, and it didn’t look like he felt really comfortable.”
Mane handed the Reds an early lead against Everton with a classy finish.
The Senegalese speedster has netted 13 times in his debut season on the Kop to become the side’s leading goal scorer.
The Reds face Bournemouth at home on Wednesday night before travelling to Stoke next weekend – a Mane is likely to be a heavy doubt for both fixtures.
Mane joined Jurgen Klopp’s side last summer in a £34m switch from Southampton.