Lejeune, 20, is a stylish defensive sweeper who has been likened to former United defender Laurent Blanc.
United, City and Real Madrid scouts have all been spotted at Istres games this season checking on Lejeune's form.
Istre president Francis Collado has confirmed the interest: "This is the time when many people come to see the players. We have a young talent, it's flattering for us. Clubs are looking for young prospects in the second division, but that does not mean they are going to sign him.
"Yes, recruiters came, but we have not had any contact. There is absolutely nothing official."