"Jason tweaked his back moving an item under his motor coach last night. This is a precautionary withdrawal for a long-standing disc issue," said Cornel Driessen, Day's physiotherapist and trainer, in a statement released by the Tour.
Day, who is No. 2 on the FedEx Cup points list following his victory at the PGA Championship earlier this month, practiced on Tuesday at Plainfield Country Club.
The Australian was scheduled to speak with the media after his pro-am round on Wednesday but cancelled that meeting as well.
Day withdrew from the 2014 BMW Championship with a back injury and struggled earlier this season with benign positional vertigo.
Dustin Johnson also withdrew from Wednesday’s pro-am because of illness. Johnson played seven holes before withdrawing but does plan to tee off on Thursday.