Lexie, played by Chyler Leigh, died of her injuries in the season eight finale earlier this month, which closed with Meredith (Ellen Pompeo), Derek (Patrick Dempsey), Cristina (Sandra Oh), Mark (Eric Dane) and Arizona (Jessica Capshaw) still stranded in the wilderness.
Asked about the fate of the stranded Grey's characters, Rhimes told TV Guide: "Just because you saw people alive at the end of the finale doesn't mean they're going to be alive when the season starts up."
Rhimes previously confessed on Twitter that the finale, which also featured the departure of Teddy (Kim Raver) from Seattle Grace, had been "incredibly hard to write".
"I did not enjoy it," she claimed. "It made me sick and it made me sad.
"[Lexie's death] was a decision that Chyler and I came to together. We had a lot of thoughtful discussion about it and ultimately we both decided this was the right time for her character's journey to end."
Grey's Anatomy cast member Jesse Williams also acknowledged that it "was kind of depressing" to say goodbye to Leigh and Raver.