ET can also confirm that Sharna Burgess is learning Arnold's routines in case she can't perform.
In a backstage video released by DWTS, the show's physical therapist tends to Arnold's knee.
"It felt like my whole leg gave out and stopped working," the 23-year-old pro dancer says, describing her injury.
"It's frustrating because it's the semi-finals and I want to dance it with Jordan. And I hope this doesn't stop me from doing that," she adds, tearing up slightly.