Zalatoris won the first leg of the playoffs at the FedEx St Jude Championship in Memphis but withdrew from the BMW Championship midway through the third round after he tweaked his lower back, dropping him to third in the standings.
"After Will's withdrawal from the BMW Championship on Saturday, his medical team determined that the source of his back pain is two herniated discs," his manager Allen Hobbs said.
"Unfortunately, this means that Will is unable to play this week at the Tour Championship.
"He is also very disappointed that he will not have the opportunity to play in next month's Presidents Cup, where he was hoping to play for Captain (Davis) Love and represent the United States."
The Presidents Cup will be held at Quail Hollow Club in Charlotte, North Carolina, from September 22-25.
"As a result of his withdrawal, if all 29 players finish 72 holes, Zalatoris will finish 30th in the FedEx Cup, which receives $500,000 in bonus money," the PGA Tour said in a statement.
The Tour Championship, the final leg of the FedEx Cup playoffs, will be held at East Lake Golf Club in Atlanta this week.
