"I have split my knee cap so from the doctors orders, he told me to rest up and that means I won't be attending the MTV Awards."
The teenager went on to express her wish that her co-stars have the "best time" at the glamorous event.
She added: "I hope that Finn, Gaten, Caleb, Sadie and Noah have the best time. I love them so dearly and I wish them the best, and I'm sending my luck with you. I'll be there in spirit.
Last year, Millie - who recently quit Twitter over homophobic memes featuring her photo - picked up the Best Actor in a Show award at the MTV Movie & TV Awards and she's up for the same accolade this year, as well as Best Kiss.
Meanwhile, Millie has become known for her kind deeds, as she previously vowed to attend the birthday party of 12-year-old fan Aaron, after he threw a party based on 'Stranger Things' only to find that none of his friends showed up.
Aaron's sad news was shared on Twitter by his older sister, and Millie - who plays Eleven - replied: "What!!!! Oh Well You can let them all know that everyone on behalf on Stranger things would've come! I think your awesome and next year I would like an invite... Please? (sic)"
- Bang! Showbiz