The French defender managed to play in his team's first preseason match in Ireland, but since then has not featured for the team.
While it is unclear exactly how the injury occured, his manager Rafa Benitez confirmed that Lejeune will be on the sidleines for a long time.
"He will need an operation, we need to decide exactly when." the Spanish coach told 'Sky Sports'
"We could lose him for a while. Bad news for us."
The player was an important part of his club's season last year, as he made 24 appearances following his transfer from Spanish side Eibar.
While it weakens the Newcastle back line, the club were able to secure the signing of Swiss international centre-back Fabian Schar from Deportivo La Coruna.
It is unclear whether the 'Magpies' are looking to sign another defender in the remainder of the transfer window, with Benitez's priority seemingly to bring in another striker.