Gabriel Boschilia will spend six months on the sidelines after undergoing surgery on a ruptured anterior cruciate ligament in his right knee.
Boschilia was carried off on a stretcher after twisting awkwardly under a challenge from Jonathan Rivierez in Monaco's 5-0 victory over Metz on Saturday.
The talented Brazilian will consequently miss the remainder of a season in which the Ligue 1 leaders are battling for trophies on four fronts.
"Boschilia has been operated on. His unavailability is estimated to be for six months," head coach Leonardo Jardim told a media conference ahead of Friday's match against Bastia.
Teenage forward Kylian Mbappe struck a hat-trick in the win against Metz, and the Monaco boss expects more to come from the exciting youngster, who has been compared to Thierry Henry.
"Mbappe is progressing to a different level. There is still room for progression," Jardim said.
"Our objective with all the young players is to make them reach the top level."
Monaco face Manchester City in the Champions League last 16 next week and will be hoping for the sort of performance Paris Saint-Germain produced to thump Barcelona 4-0 on Tuesday.
Jardim feels the result proved the strength of Ligue 1, but added for now his focus is simply on beating Bastia.
"PSG's victory yesterday is yet another demonstration of the quality of French football," he said. "Every day I defend the French league - it is very competitive with a lot of good players.
"We are working to prepare for the match against Bastia, not for City. The best preparation for a Champions League game is to win the one ahead of it.
"We have recovered well this week. It is good for us before the start of another string of important matches."