Ederson out for rest of season
The Brazilian midfielder collapsed with what looked like a hamstring injury during Sunday's 3-2 win at Udinese.
But scans on Tuesday revealed tendons in his right thigh had become detached and surgery was the only option.
The 28-year-old suffered a similar injury to his left thigh in August 2010.
"Unfortunately, the lad suffered the same lesion as in the past, but this time in his right thigh," doctor Roberto Bianchini told Lazio Style Radio.
"He will need surgery and he asked for the same specialist who operated on him in Lyon. We agreed to have the surgery there.
"It will be a long lay-off, about five or six months. In the initial period of recuperation he'll have to stay completely immobile with a plaster cast then he can start on light work in the pool.
"This is major surgery. The way the incident occurred and the pain he felt made us immediately worry this was not something banal."
Ederson has scored one goal in 15 appearances for Lazio this season.