Diller will miss her July 18 "Tonight Show" appearance marking her 90th birthday after injuring her back when she "twisted the wrong way," her manager, Milt Suchin, told The Hollywood Reporter.
"The doctors say to rest and let it heal. ... She's in great spirits," he said.
"It's very disappointing. She was going to do a stand-up bit but she really can't stand up properly. She's not immobile. She is functioning normally."
Suchin said Diller is expected to miss a month or so of appearances, including one with the Television Critics Association this week, the Reporter said Thursday.
"She's a trooper. She's a phoenix, she rises. She's gone through a lot worse.
"If (the injury) mends sooner, we're out there sooner," he said. "There's no lack of demand for her services."