Newcastle have confirmed Florian Lejeune has undergone successful knee surgery to repair an anterior cruciate ligament tear.
The club have not yet put a timescale for the defender's return to action from an injury suffered during pre-season training earlier this summer.
Lejeune, 27, posted a picture on social media with the caption: "My knee operation was a success. Now it's time to rest and recover."
"I was speaking with Florian before and after his operation," Newcastle manager Rafael Benitez told the club's website.
"The doctor says it has gone well and he seems to be in good spirits. I join everybody at the club in wishing him a speedy recovery."
Lejeune was signed from Eibar last summer and the Frenchman made 24 appearances last season.