The Brazilian defender went down early in the second half in Real Madrid's opening La Liga game of the season against Athletic Bilbao on Saturday.
Real Madrid won the encounter at the San Mames 2-0 but their victory came at a cost, with their central defender set to spend a significant chunk of the season on the treatment table.
Real Madrid also lost goalkeeper Thibaut Courtois to an ACL injury last week and the Belgium international is set for a long layoff this season.
A club statement on Sunday read: “Following tests carried out on our player Eder Militao, he has been diagnosed with a torn anterior cruciate ligament in his left knee.
“The player will undergo surgery in the coming days.”
Manager Carlo Ancelotti was fearing the worst when asked after Saturday night’s game.
“It’s a knee sprain, it doesn’t look good,” he said. “We will assess it in the next few hours. We can’t rule out it being a serious injury.”
Militao's injury leaves Real Madrid with just three fit senior central defenders; Nacho Fernandes, David Alaba and Antonio Rudiger.
Militao has made 143 appearances for Real Madrid since joining from FC Porto in the summer of 2019 and his list of honours at the Santiago Bernabeu includes two La Liga titles and one UEFA Champions League.
