Striker Radamel Falcao picked up a possibly serious knee injury as Monaco reached the French Cup last 16 with a 3-0 away win over amateurs Chasselay Monts d'or Azergues on Wednesday.
The Colombia striker opened the scoring in the 29th minute but was carried off the pitch on a stretcher shortly before the break after being tackled by Chasselay defender Soner Ertek.
"We lose Radamel Falcao. It's a big blow. I think it is serious. The news is not reassuring," captain Jeremy Toulalan told Eurosport.
Monaco said in a statement that Falcao had sustained a knock on his left knee and they were awaiting further checks to assess the injury.
Striker Emmanuel Riviere netted the two other goals in a one-sided encounter played at Lyon's Gerland stadium as fourth division team Chasselay, with former Monaco and France winger Ludovic Giuly in their ranks, failed to pose a threat.
Later on Wednesday, Ligue 1 leaders Paris St Germain were at home to Montpellier.