Bielsa will be forced into a change on the left side of his defence for the first time this season after Douglas sustained a hamstring strain in Tuesday’s 1-0 win at Hull City.
The setback is the latest in a growing line of injuries which have affected Bielsa’s squad since the start of the season but Douglas, who holds an ever-present record in the Championship, could be available for Leeds’ trip to Blackburn Rovers after the international break.
United have overcome the loss of Kemar Roofe, Patrick Bamford, Pablo Hernandez and Gaetano Berardi to hold onto first place in the division and Douglas will join those four on the sidelines as Bielsa looks for a seventh win of the term this weekend.
Douglas limped from the pitch at Hull eight minutes from the end of Tuesday’s derby and was sent for scans yesterday.
A title-winner with Wolverhampton Wanderers last season, he has been Bielsa’s first-choice left-back ever since arriving from Molineux in July.
Tom Pearce and Stuart Dallas are the most likely candidates to replace him against Brentford.
Speaking this afternoon, Bielsa said: “He won't be available for this coming game. I think he will be available to play after the international break but I couldn't assure you of that.
“It's a difficulty not to have all players available but we have the solution of Dallas and before Douglas arrived, he played an important role as a left-back. We also have Tom Pearce and without minimising the importance of Douglas we are always prepared."