Florian Lejeune has undergone knee surgery in Italy – for the second time in eight months.
Lejeune ruptured the anterior cruciate ligament in his left knee against Crystal Palace earlier this month.
The Newcastle United defender had suffered the same injury in his right knee last summer.
Rome-based surgeon Prof Pier Paolo Mariani has again operated on the 27-year-old, who has posted an update on Twitter from his hospital bed after a successful operation.
Lejeune tweeted: "My operation went well, now it's time to be patient, work hard and come back stronger. Thank you Pr. Mariani."
Rafa Benitez hopes Lejeune – who had made his first-team comeback in January's FA Cup tie against Blackburn Rovers – will be back in "four or five months".
United's manager said: “Now we have an important player who was working so hard to be fit, now he has to start again.
“He’s a very important player for us in that position, playing out from the back.
“I think he’s a strong character. He was working so hard to get fit, and he will have that same determination this time.”