The 28-year-old suffered a torn tendon in Sunday's match against Udinese, and the club have revealed that he will need to undergo surgery.
The Brazilian was out of action for seven months in 2010 after suffering a similar injury to his thigh.
Club doctor Roberto Bianchini is quoted on Sky Sports as saying: "Unfortunately the lad has suffered the same injury he has suffered in the past but this time to his right thigh.
"It is an injury which will need surgery, much the same as with his other leg."
Ederson has made 15 appearances for Lazio this season, scoring once.