But McIlroy, the only three-time FedEx Cup champion, couldn't have been happier.
McIlroy revealed after the first round that he injured his back while stretching at home Tuesday. He said near the end of most seasons, he has to manage the right side of his body because of tightness.
"My right side always gets pretty tight, my rib cage, intercostals, lats, like, all the way down, right hip," McIlroy said. "On Tuesday morning, I felt a little tight, and I went into my gym at home, and I just sort of foam-rolled and stretched. I went to grab something and my whole right side just completely seized up, spasm."
McIlroy said he spent a couple of hours with his athletic trainer before flying to Atlanta. He felt better after getting additional treatment, so he went to the gym to try to get in some light work.
"I was at the bottom of a squat, a body-weight squat, and my whole lower back spasmed, seized up," McIlroy said. "I couldn't move. I honestly couldn't address the ball this time yesterday. So to get to where I am today is good."
McIlroy said he arrived at East Lake Golf Club at about 7:45 a.m. ET Thursday, about six hours before his scheduled tee time with Jon Rahm. He took an ice bath and stretched. He hit 20 wedge shots before 10 a.m., which he said were the first swings he'd taken since Sunday's final round of the BMW Championship outside Chicago.
The four-time major champion was stretching and grimacing while hitting drives on the practice range only minutes before his tee time. His caddie, Harry Diamond, gave him three ibuprofen before they headed to the first tee.
"I felt OK, so then just thought I would give it a go," McIlroy said. "So I was always going to tee off. It was just a matter of how I felt on the course. And it got progressively a little tighter as I went, but it will hopefully get loosened up here and just another 20 or 18 hours of recovery and go again tomorrow."
After finishing third in FedEx Cup points this season, McIlroy started the round at 7 under in the staggered scoring format. He made bogeys on Nos. 2 and 8 and made the turn at 2-over 37. He picked up four birdies and two more bogeys on the back to get to even.
He is tied for seventh, 3 strokes behind co-leaders Morikawa, Bradley and Viktor Hovland.
Despite the injury, McIlroy ranked second in the field in driving distance (315.9 yards) and was eighth in strokes gained: putting (1.461). He hit seven of 14 fairways and 11 of 18 greens.
"I hung in there and I just felt like if I could get through today, it's better than it was yesterday, hopefully tomorrow's better than it was today, and just sort of try to keep progressing," McIlroy said. "So the fact that I'm only going to be like, three or four off the lead, I'm over the moon about."
McIlroy tees off at 1:27 p.m. Friday.