Professional "Dancing with the Stars" dancer Derek Hough injured his back and may not perform on the Los Angeles show Monday, UsMagazine.com said. If the 28-year-old dancer is, indeed, sidelined, his partner, "Glee" star Amber Riley, will dance with substitute Henry Byalikov. "[Hough's] back is severely injured -- he had a pre-existing injury," an insider told UsMagazine.com. "He hurt his back dancing with Shawn Johnson [during Season 15 in late 2012,] and he re-injured it dancing with Amber. He is determined to push through but not sure he'll be able to."