Djimi Traore is facing up to a prolonged spell on the sidelines after damaging knee ligaments.
The experienced full-back fell awkwardly during Monaco's meeting with Paris St Germain on Saturday and could be out for the remainder of the season.
It has already been decided that he will require an operation to resolve the problem, with surgery due to be carried out on Thursday.
The setback comes as a major blow to the 30-year-old defender as he is set to see his current contract expire next summer.
Any hopes he had of playing his way to a new deal appear to have now been shattered.
Coach Guy Lacombe could also have done without losing such an important member of his side at a vital stage of the season.
Monaco have struggled to recover from a slow start to the campaign and occupy a standing just outside the Ligue 1 relegation zone at present.