Hideki Matsuyama withdrew from the BMW Championship prior to the start of the second round on Friday due to a lower back injury.
Matsuyama has dealt with back and neck injuries for the past few years.
The 2021 Masters champion missed a short birdie putt on the 18th hole and carded a 5-under-par 67 on Thursday at Castle Pines Golf Club in Castle Rock, Colorado.
He sat one stroke back of leader Keegan Bradley after the first round.
Matsuyama, 32, recorded his 10th PGA Tour title of his career by winning last week's playoff opener, the FedEx St Jude Championship.
That victory allowed him to soar into third in the FedEx Cup standings.
Matsuyama, of Japan, also was a bronze medalist at the Paris Olympics.