Midfielder Douglas, who played at right-back over the latter part of the season, and goalkeeper Lucas will both officially leave Elland Road on June 30.
Manager Simon Grayson said: "I've said all along that I'd evaluate the squad at the end of the season and this is part of that process.
"I'm looking to strengthen what we have here and, while players will be coming in, departures are also inevitable as we look to prepare in the best manner possible for next season."
Douglas spent four seasons at Elland Road, making 166 appearances, after initially joining on a season-long loan from Blackburn. His move was made permanent in August 2006.
Lucas has made 24 first-team appearances since arriving as cover for Casper Ankergren in August 2007.