"Momo has undergone further tests this morning which have confirmed damage to the retina in his right eye," said a club spokesman.
"The injury is currently affecting his eyesight. Further consultations with eye specialists have been arranged for when the player returns to England."
Benfica's Beto, whose challenge caused the injury was apologetic.
"My boot caught Sissoko in the face and it opened a big wound around the eye," he said. "I want to apologise - I did not mean to injure him.
It was just a typical 50-50 midfield situation that happens all the time; you have to contest everything. It happened, but I did not mean to harm him and I hope he recovers quickly."