You can't keep DONNY OSMOND down – even with a busted behind!

The brave 55-year-old is battling back from a debilitating injury to his derriere, but he's determined to get back onstage with his sister MARIE ASAP to save their Las Vegas show.

Donny severely tore his gluteus maximus muscle when he leapt onto a piano during a performance at the Flamingo in early July.

The tush tear was so serious the entertainer required major surgery – and he was warned that it would take months of rehab before he was ready to perform again.

Recent photos, taken during a family cruise to Bermuda by daughter-in-law Shelby and posted on Instagram, show him confned to a wheelchair.

But Donny's ready to get his butt back in gear now, and plans to take the Las Vegas stage alongside Marie starting Sept. 3. He also intends to join her for a tour of 15 shows in 12 states beginning Dec. 1.

While his hind quarters have been healing, several people have flled in for him, including his and Marie's brother MERRILL their nephew DAVID, and “Dukes of Hazzard” veteran JOHN SCHNEIDER. But fans want what they paid for – Donny!

“He's determined the show will go on,” said a source, “no matter how painful it will be.”

As the ENQUIRER reported in our Aug. 5 issue, the superstar siblings had been battling ever since Marie's Hallmark Channel talk show was canceled in early July.

But Marie realized she needed Donny as a partner – both onstage and as a brother. “Now they're closer than ever,” added the source.

“Marie wants to make it as easy as possible for Donny to return.”