The world number six had been due to tee off with South Africa's Louis Oosthuizen and England's Justin Rose at Valhalla on Thursday.
However, the 36-year-old's latest attempt to break his major duck has been curtailed due to injury and he has been replaced by first reserve John Huh in the field for the final major of the season.
"Matt Kuchar has withdrawn (back pain). That opens a spot for first alternate John Huh," the US PGA Championship posted on its official Twitter account.
Kuchar, a seven-time winner on the PGA Tour, is now sure to face questions about whether he will be fit enough to play for the United States in the Ryder Cup at Gleneagles next month.
Back trouble also caused doubts over Tiger Woods' participation after his withdrawal from last week's WGC Bridgestone-Invitational, but the 14-time major winner managed to tee off.