Dufner has been severely hampered by two bulging disks in his neck since April and was eight over par for the day, having triple-bogeyed the par-five 10th, when he decided to pull out of the year's final major.
He shook hands with his playing partners, fellow American Keegan Bradley and South Korean Yang Yong-eun, who are both former PGA Championship winners, before leaving the course.
Dufner had to limit severely his preparation for the tournament after having an epidural and being advised by doctors to take as much rest as possible.
"They like you to rest seven to 10 days after that," he told reporters on the eve of the PGA Championship. "I haven't really been able to do that, so I'm trying to rest, keep my reps down. I feel sore. I get a little fatigued quicker than usual.
"It's an arthritic, degenerative issue that I'm probably going to be dealing with for a good bit. It's just something that's not going to change. I'm going to have to deal with it."
Dufner, who has been troubled by the injury since the Masters in April, said he would not be healthy until he was able to take a break from the game of six-to-eight weeks.
"Not sure when that's going to be, but I'll get through it," the 37-year-old said. "Obvious reasons for playing this week; defending (the title) is pretty important to me."
Dufner won his first major with a two-shot victory in last year's PGA Championship at Oak Hill.
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- PGA Championship