Buck was placed on the 15-day disabled list and catcher Brayan Pena was recalled from Triple-A Omaha to take his roster spot for the Royals' game against the White Sox on Sunday.
Manager Trey Hillman says he has no indication how long Buck might remain in the hospital or whether the "slight herniation in his lower back" might require surgery.
Buck was due to lead off the bottom of the third inning when the problem appeared to flare up. He descended the dugout steps with assistance and in obvious discomfort.
He was hitting .226 with three home runs and 19 RBIs while sharing time behind the plate with Miguel Olivo.