'Dancing With the Stars' Pro Peta Murgatroyd Out for Season 21

— -- Longtime "Dancing With the Stars" pro Peta Murgatroyd will not be able to compete in the upcoming season 21.

She told People magazine that she needs surgery, scheduled for mid-September, and will be out for at least "six to eight weeks." She was to be Andy Grammer's partner.

Watch "Good Morning America" tomorrow at 7 a.m. ET for the announcement of the full celebrity cast and pro pairs!

She recently knew something was wrong and said, "I wanted to check it out properly and make sure I was good to go for the season. I Got my MRI results back and it was a really bad diagnosis. It showed I had 4 torn ligaments and a floating piece of bone at the back of my ankle."

Telling her partner that he would be dancing with someone else "was awful," Murgatroyd, 29, said.

"He had specifically asked to be partnered with me and I felt like I was letting him down," she added. "It's super frustrating but I'm going to be fine and I'm going to be back in week eight or nine of the show."

"Dancing With the Stars" is back Sept. 14.