Veteran pop star Donny Osmond has been forced to sit out his Las Vegas cabaret show after tearing his buttock muscle during a performance. The 55 year old was onstage with his sister Marie at the Flamingo hotel and casino recently when he leaped on a piano and aggravated an existing injury, according to LasVegasSun.com. Reports suggest Osmond had to undergo surgery to repair his gluteus maximus and is now resting up. His brother Merrill has replaced him onstage with Marie and the Globe reports another singer is being groomed to stand-in for Donny on a more permanent basis until he's fully recovered. His fun-loving sister says, "I've always known he's been a butt - but now he's proven it."