ET can confirm that Arnold injured her knee during rehearsals on Sunday, threatening her ability to perform on Monday night's show.
A DWTS source tells ET, "Lindsay injured her knee during camera blocking rehearsal Sunday morning while working on a number with the troupe. Lindsay is hopeful she’ll be cleared to dance tomorrow, but it will likely be touch and go until showtime Monday."
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.
Arnold and Fisher finished at the top of the judges' leaderboard last week, with a perfect score of 60 points. They are slated to perform two dances Monday night. The first is an Argentine Tango set to the song "Brother" by NEEDTOBREATHE (featuring Gavin DeGraw). The other is a recreation of an iconic dance from a past season, in this case, Mark Ballas and Paige VanZant's Jive to Tina Turner's "Proud Mary."
“It was such a good night and it could have not happened to a more deserving person,” Arnold told ET about Fisher. “I mean that. It has been a crazy eight weeks. I am so, so grateful for every moment that we have had together, and having tonight was such a good night for us, and I am really, really proud.”
For more on Arnold and Fisher, watch the video below: