Nathan Collins picked up an injury on international duty (Picture: Getty Images)
The 22-year-old suffered the injury while playing for Republic of Ireland against Netherlands last week and has already returned to his club.
The Evening Standard report that the injury is not thought to be a serious ligament problem, but will be enough to keep the centre-back out for at least two weeks.
That will see Collins miss the 5.30pm meeting with Arsenal on Saturday and a home game against Luton Town on 2 December.
Bees boss Thomas Frank will hope that he is back for a busy schedule over the rest of December, though, with trips to Brighton and Sheffield United on 6 and 9 December respectively.
Brentford are dealing with a number of fitness issues, with the likes of Rico Henry, Aaron Hickey and Kevin Schade all still out, while Ivan Toney remains banned for betting offences.
However, there could be returns for Mikkel Damsgaard, Keane Lewis-Potter and Joshua Dasilva after the international break.
Arsenal are hoping to have Martin Odegaard, Ben White, Gabriel Jesus and Emile Smith Rowe available again after injury.
Thomas Partey and Jurrien Timber remain on the long-term injured list, while Fabio Vieira is suspended after his red card last time out.
For more stories like this, check our sport page.
The Bees host the Gunners on Saturday.