The 33-year-old Irishman has been with the Bees for four years, making 187 appearances, and only signed a new contract at Griffin Park in February.
He was also a key figure in the side which reached the Championship play-offs under previous manager Mark Warburton last season.
But Douglas was absent from the squad beaten at Luton in Tuesday night’s pre-season friendly, after which Dijkhuizen revealed he is unlikely to play for the club again.
“He’s not part of the squad,” said Dijkhuizen. “He’s training separately.
“We decided with (directors of football) Rasmus Ankersen and Phil Giles that it’s better to move on, for his future, and for Brentford.”
New signing Philipp Hofmann was on target on his first Bees start but goals from Luke Wilkinson and Calvin Zola secured a 2-1 win for Luton.