The 47-year-old rocker's Web site says Michaels was urged to go to the hospital as he was preparing to take the stage in San Antonio on Sunday night.
He had been having severe stomach pains all day "and what was assumed to be a severe case of the flu."
The statement says Michaels had emergency surgery early Monday and is now "doing well and beginning the recovery process."
Michaels was the frontman for the '80s hair band Poison.
