Derek Hough

Oh no!

Derek Hough could be off Dancing With the Stars for part of the season, or the whole rest of the season, and his celebrity partner, Olympian gold medal winner Nastia Liukin, might be getting a new partner, E! News has learned.

The fan-favorite pro broke a toe, sprained his ankle and suffered ligament damage while rehearsing Monday night, according to sources, and is awaiting final test results that will determine how long he is off the show. Nastia will be rehearsing with Sasha Farber, one of the troupe members who also subbed in for Derek's 10th Anniversary Special performance, until the extent of Derek's injuries are determined.

"It sucks. It's unfortunate," Derek's friend and fellow DWTS pro Mark Ballas tells E! News. "Hopefully he's going to be OK. The MRI results will come in [today] probably and we'll know more, but we're just praying that everything's OK."

ABC is not yet commenting on Derek's DWTS status.

"Nastia, my heart goes out to her, just passed midway in the season, having to change pros," fellow pro Sharna Burgess told us on the red carpet for Dancing With the Stars' 10th Anniversary Special on Tuesday night. "We build a language between each other on how we work with each other and that's what enables us to be able to do what we do every week with all the pressure and the stress, trying to do things that are unnatural for these people. And for her to now have to change pros, it's like changing a language, changing a connection and changing trust. It's really difficult. She's incredibly talented, I know she will handle it beautifully, but it's not fun."

Derek, who has been pulling double duty with DWTS in Los Angeles and the Spring Spectacular show with Laura Benanti and the Rockettes in New York City, injured himself while rehearsing with Maria Menounos for the anniversary special after Monday night's show. He spent the night in the emergency room accompanied by his close friend Mark Ballas. "I do, in someway, believe this was the universe saying Derek you need to take a break," Sharna said of the fan-favorite's busy schedule.

Spring Spectacular is now playing at Radio City Music Hall, and it is not yet known how Derek's injuries will affect his schedule there as well. He sat out the performance on Tuesday with Taylor Frey stepping in for the Emmy-winning professional dancer.

Yesterday, a source told E! that Derek had most likely broken a toe and possibly severely sprained his ankle and may have suffered ligament damage. He posted the above Instagram photo last night.

"I was with him until all hours of the night…Derek's an optimist," Mark told us. "He's always up and he's always bringing the people around him up. His energy is always positive."

Fortunately, Nastia has some experience dancing with someone other than Derek as she's actually been rehearsing with another DWTS' dancer all-season due to Derek's Broadway commitment. For half of the week, Nastia rehearses with troupe member Henry Byalikov in Los Angeles. "He's great, but it's definitely different than dancing with Derek," Nastia once said on the show.

Dancing With the Stars airs Mondays, 8 p.m. on ABC.