Donny Osmond nurses his tush while brother Merrill sings with Marie

When one Osmond goes down, another one is ready to take his place.

That's what's happening in Sin City, as Merrill Osmond is pinch-hitting for his brother Donny by performing alongside their sister Marie in Donny & Marie's headlining show at the Flamingo Las Vegas.

Donny suffered a torn gluteus maximus — yep, he broke his butt — about a week ago, aggravating an existing injury while leaping on the piano in a performance, according to the Las Vegas Sun. Donny tried to perform anyway, but hasn't been onstage since last Tuesday.

On Saturday, Marie told the audience that Donny's butt muscle tore away from bone, and he's undergone surgery to repair the damage.

Older brother Merrill Osmond subbed for Donny on Saturday, performing duets with Marie and performing a couple of solos. Merrill is scheduled to perform with Marie through the end of July, and another singer to be named later may fill in after that, the Sun reported.

Marie couldn't resist a few jabs at her brother's plight, at one point joking, "I've always known he's been a butt, but now he's proven it."