ABC's medical series "Grey's Anatomy" has lost two key cast members, but the show is reportedly far from being in trouble. Isaiah Washington, 43, who played surgeon Preston Burke on the show, was told Thursday by "Grey's" creator Shonda Rhimes he would not be invited back for the next season, USA Today reported. Kate Walsh, who plays OB\/GYN Addison Montgomery, has also left the show to star in "Private Practice," a spin-off that will air on Wednesdays this fall. But the show's executives have revealed they plan to add several cast members during the fourth season, including an intern, Meredith Grey's half-sister Lexi, played by Chyler Leigh. They also said they are seeking a new recurring male character to compete with Patrick Dempsey's Dr. Derek Shepherd.