The 21-year-old Belgian, the younger brother of Palace record signing Christian, has made one appearance for the Eagles since signing in August.
Palace manager Alan Pardew said the injury news was "disappointing" for both club and player.
Pardew, whose team visit Everton in the Premier League on Friday, said: "His meniscus on his knee is the problem."
Crystal Palace striker Jonathan Benteke has been ruled out for between three and four months with a knee injury.