The 19-year-old came off with an ankle injury during the Ligue 1 club's Europa League victory over Qarabag on Thursday.
Further tests on Friday revealed that he will be out of action until the end of November.
Lemar joins Adama Traore, Fabio Coentrao, Tiemoue Bakayoko, Fares Bahlouli, Helder Costa and Almamy Toure on Monaco's injury list.
Leonardo Jardim's depleted squad travel to Reims on Sunday.
Restore Data
| Team | P | W | D | L | F | A | GD | PTS | |
| 1 | Paris Saint-GermainPSG | 36 | 27 | 3 | 6 | 86 | 36 | 50 | 84 |
| 2 | Lens | 36 | 23 | 9 | 4 | 62 | 28 | 34 | 78 |
| 3 | Marseille | 36 | 22 | 7 | 7 | 66 | 37 | 29 | 73 |
| 4 | MonacoMonaco | 36 | 19 | 8 | 9 | 69 | 54 | 15 | 65 |
| 5 | Lille | 36 | 18 | 9 | 9 | 62 | 42 | 20 | 63 |
| 6 | Rennes | 36 | 19 | 5 | 12 | 65 | 38 | 27 | 62 |
| 7 | Lyon | 36 | 17 | 8 | 11 | 61 | 44 | 17 | 59 |
| 8 | Clermont | 36 | 15 | 8 | 13 | 40 | 47 | -7 | 53 |
| 9 | Nice | 36 | 13 | 13 | 10 | 42 | 34 | 8 | 52 |
| 10 | Lorient | 36 | 14 | 10 | 12 | 50 | 50 | 0 | 52 |
| 11 | Reims | 36 | 12 | 15 | 9 | 44 | 39 | 5 | 51 |
| 12 | Montpellier HSCMontpellier | 36 | 14 | 5 | 17 | 60 | 58 | 2 | 47 |
| 13 | Toulouse | 36 | 12 | 8 | 16 | 48 | 55 | -7 | 44 |
| 14 | Brest | 36 | 10 | 11 | 15 | 41 | 51 | -10 | 41 |
| 15 | StrasbourgStrasbourg | 36 | 9 | 12 | 15 | 49 | 56 | -7 | 39 |
| 16 | Auxerre | 36 | 8 | 10 | 18 | 33 | 59 | -26 | 34 |
| 17 | NantesNantes | 36 | 6 | 15 | 15 | 35 | 53 | -18 | 33 |
| R | Troyes | 36 | 4 | 11 | 21 | 43 | 78 | -35 | 23 |
| R | Ajaccio | 36 | 6 | 5 | 25 | 22 | 71 | -49 | 23 |
| R | Angers | 36 | 3 | 6 | 27 | 31 | 79 | -48 | 15 |
| > Ligue 1 Full Table | |||||||||
