New Chelsea signing Malo Gusto suffered a hamstring injury for Lyon on Wednesday during their French Cup match against Lille.
Highly-rated defener Gusto, 19, joined the Blues from Lyon in a £26.3million move on a seven-and-a-half year deal last month.
As part of the transfer it was agreed the France under-21 international would stay on loan with the Ligue 1 side for the remainder of the 2022-2023 season.
However, in Wednesday's clash against Lille, Gusto was taken off after just 33 minutes and was seen holding his left hamstring as he left the pitch.
Following the game, Lyon boss Laurent Blanc explained the severity of the injury.
'It’s a pain he’s been feeling for some time,' he told L'Equipe. 'It’s not a big problem, but it bothers him physically and mentally.
'We will try to treat him well… I don’t think it will take too long, but it has to be done.'
Before making a move for Gusto, Chelsea had identified the right-sided defender as a player who can provide back-up to Reece James with the 23-year-old having struggled with injury this season.
The Blues were met with initial resistance to their interest from Lyon with president Jean-Michel Aulas publicly declaring Gusto 'will continue to play' for the French side until the end of the season, but they eventually got their man.
He joined Christopher Nkunku as a pre-agreed summer arrival after Chelsea also finalised a £53m deal to sign the France forward from RB Leipzig.
Gusto took to Chelsea's official Instagram page after the move was completed to address the club's supporters. He said: 'Hey Chelsea fans, I am very happy to be here. Thank you for your messages and I will see you soon.'
The 19-year-old has made 55 appearances for Lyon's first-team since making his debut as a 17-year-old in January 2021, and he has become a regular feature in their starting line-up, despite his age.
He has made 18 appearances in all competitions for Lyon this season, with the French club currently ninth in Ligue 1.