Lucas' exit was expected with his first-team opportunities due to be limited next season, but Douglas is leaving after the club and his representatives agreed that a new contract would be 'exceptionally difficult' to finalise.
The Republic of Ireland midfielder, 27, was signed on a three-year contract in 2006 while Leeds were in the Championship, and he is likely to have been among the higher earners at the club.
As many as four other clubs, a number in the Championship, have already shown interest in his signature, and Leeds have decided against involving themselves in lengthy contract talks which they believe would fail.
A statement from Leeds reads: "In respect of Douglas, the club and his representatives felt it was going to be exceptionally difficult to agree terms that would be acceptable to both parties."
Douglas is leaving the Whites after completing four seasons, including one on loan from Blackburn Rovers, and more than 160 appearances for Leeds, a spell in which he saw two play-off final defeats and relegation from the Championship.