DEREK HOUGH returned to "Dancing with the Stars" last night after missing last week due to a neck injury.  (In his place, MARK BALLAS danced with SHAWN JOHNSON.)  In a post on his website, Derek says his doctors have cleared him to finish out the season, but eventually, he's going to need neck surgery.

He says, "It's feeling much better, but I still have to be cautious, because it only takes one small wrong move for me to be back in agony."  (You can read his whole message at DerekHough.com.)

Derek tells "Extra", "I have a herniated disc and two bones spurs, which basically is extra bone growth pushing to the nerve of my spinal cord."  He adds that his doctors couldn't believe he messed himself up that bad just by dancing.  He says, "They asked me, 'Have you been in any car accidents?  Ski accidents?  Something traumatic?'"  (There's video at ExtraTV.com.)