Hough has been partnered with Olympian Shawn Johnson for the show's current all-star edition.
UsMagazine.com said he aggravated a pre-existing neck injury during the program's Oct. 30 broadcast and couldn't dance on Monday night's show, so Ballas subbed for him.
"Derek's injury is really serious," a show insider told UsMagazine.com.
"It's likely Mark will finish out the season for him," the source said. "Derek is so disappointed but very badly hurt. He needs surgery."
Swim Week Miami Beach 2014 [PHOTOS]