"I have a bulging disc on my vertebrae," he explained to Us Weekly as he iced his neck post-show. "I've had it for a couple years. I need surgery on it. It flares up and it did today unfortunately. It immobilizes me but we dance through it on adrenaline."
The pain was too much to push through Monday night, however.
"Derek's injury is really serious," a show insider tells Us, explaining that Hough will not be dancing Monday. (Mark Ballas is slated to fill in for him, instead).