Sissoko was taken to hospital with a suspected dislocated shoulder and Rafa Benitez admitted to fearing the worst.
"We are keeping our fingers crossed but I think it's serious. I can't say anymore at this moment because we are waiting for news," said the Spaniard.
"We need all of our players and we know that Momo gives us wonderful energy in the middle of the field, but it's clear we could be without him for a long while, at least one month.
"Momo is unlucky with injuries, especially with the kind of injuries he has received, the eye last season and now the shoulder. They are not very common injuries.
"I haven't seen the replay of the incident but it was an accident. It was clear something was wrong. Momo is a player who doesn't like to stay on the ground."