Sierra, 21, is accused of hitting a man on the head with a glass at a Tampa nightspot in April, police said. Officers found a "small amount of cocaine" while searching her during booking at the county jail, police said.
She has pleaded not guilty to charges of aggravated battery and cocaine possession.
Sierra entered a rehab facility in July for treatment of unspecified problems and will likely be there for several months, her attorney, John Fitzgibbons, said in court Wednesday.
"I don't want to get into the precise issues, but it is very helpful to her," Fitzgibbons said.
A judge rescheduled Sierra's trial from October to November.
Sierra was among the top 10 contestants on the Fox talent show in 2005.