Speaking to E! Online, Carlton Cuse said that Grace is 'too busy' to reprise her role as Shannon Rutherford.
"We would love to have Maggie Grace back, but she's very busy - she's got a movie career," he said.
"We would love to work it out and have her back on the show, and we've talked to her; we're just trying to get that worked out."
Shannon was last seen in season two when she was accidentally shot by Ana Lucia and died in Sayid's arms shortly afterwards.
Recent confirmed returnees for the final season - scheduled to premiere Tuesday, February 2 on ABC - include Dominic Monaghan, Harold Perrineau and Cynthia Watros.