The Belgian hit-man, who signed with The Lions for an undisclosed fee last summer from Dutch club Vitesse Arnhem, has been sidelined with the problem in recent weeks.
The 30-year-old has now been told that he needs surgery to repair the problem, with the striker likely to be sidelined for the rest of the campaign.
Meanwhile, defender Matthew Rees has returned to the South East London club following a month-long spell with Conference club Dagenham & Redbridge.
One player that has left The New Den is Republic of Ireland youth international Tim Clancy, who has rejoined former Millwall striker Steve Claridge for a second loan spell with Dr Marten's League Premier Division leaders Weymouth.
