Chelsea defender Kalidou Koulibaly is a major doubt for this week's Champions League tie against Red Bull Salzburg.
The Senegalese, who joined this summer from Napoli, missed Saturday's clash with Manchester United and looks set to be absent again after missing Monday's open training session.
It is a blow for Graham Potter's side who will qualify this week should they win in Austria.
Chelsea, already shorn of N'Golo Kante and Reece James for the foreseeable future, will be hoping the knee injury is nothing serious entering into a hectic period ahead of the World Cup in November.
The mounting injuries leave the Chelsea manager with a gradually thinning squad as they look to cement their place in the top four ahead of the season breaking on November 13.
Koulibaly has proven to be an important player in Potter's side since the manager arrived from Brighton at the start of September.
Having been left out of the starting XI for the fixture against the Austrian outfit at Stamford Bridge, he played 90 minutes in both games against AC Milan.
However, Potter revealed that towards the end of the Brentford game he 'felt something' in the back of his knee and as a result could not be risked for United.
It appears that Chelsea will now be without him for a second consecutive game as they look to get back to winning ways after two draws in the Premier League.
Wesley Fofana, meanwhile, continues in his rehab after injuring his knee against AC Milan at the beginning of the month.
The defender was forced off before half-time against the Italian champions in one of only two appearances he's made under Potter so far.
It is hoped the former Leicester defender could be fit in time for the World Cup.