"The sores are especially bad where her leg joins her prosthetic limb, but she's determined not to give up," a source told the U.K.'s Closer magazine. "The show's rehearsals have left her in a lot of pain."
Mills was hit by a police motorcycle while crossing a London street in August 1993 -- crushing her ribs and giving her a punctured lung. She also needed a metal plate in her pelvis and had her left leg amputated below the knee.
"We don't know how good she's going to be as a dancer," Dancing with the Stars executive producer Conrad Green said in the March 5 issue of TV Guide. "But she's a very fit young woman. She's run long distances. She's a lot fitter than I am."
Green said Dancing with the Stars judges Carrie Ann Inaba, Len Goodman and Bruno Tonioli have also been instructed to treat Mills "exactly the same as any other dancer," with the one major exception being she won't be marked down for something she can't physically do.
"When you put your leg out, you're supposed to point your toe like a ballerina," Green told TV Guide. "Obviously, you can't do that with a prosthetic leg."
If the prospects of trying to two-step with a prosthetic limb weren't bad enough, Mills is also currently involved with the pending divorce from her estranged husband, former Beatle Paul McCartney.
"Heather has been through hell recently and it's great for her to focus on something other than her daughter and the divorce," the source told Closer. Added Green, "I don't think [the pending divorce] will be an issue for our show at all."
Green said he approached Mills about appearing on Dancing with the Stars because he wanted to "recruit a disabled dancer who defies expectations."
"For the past 10 years, she's been raising money for the victims of land mines. She's hugely committed to spreading the word about what you can do... even if you have a disability," Green told TV Guide. "The opportunity to be on such a big show and prove that point was something [Mills] didn't feel she could turn down."