McIlroy is the only three-time winner of the FedEx Cup after overturning a six-shot deficit in the final round of the same tournament last year.
He said he is “over the moon” to be placed where he is considering the spasms.
“I was at the bottom of a squat, a body-weight squat, and my whole lower back spasmed, seized up. I couldn’t move. I honestly couldn’t address the ball this time yesterday,” McIlroy said.
“So, yeah, I mean, 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 is sitting on seven under par in tied seventh after he finished the day with the same score as he started with, hitting four birdies and just as many bogeys.
“So I was always going to tee off. It was just a matter of how I felt on the course,” McIlroy said.
“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.”
There is a three-way tie for the lead between Collin Morikawa, Keegan Bradley and Viktor Hovland on 10 under par.
England’s Matt Fitzpatrick is even with McIlroy in seventh spot while Tyrell Hatton is one stroke behind the pair.
McIlroy is due to tee off his second round today at 6.27 BST.