"I thought conditions were just as tough, if not tougher this morning, as they were yesterday", said McIlroy, whose only major victory when trailing after 36 holes came when he was two behind in the 2012 US PGA.
American Justin Thomas (69) and South African 2010 British Open champion Louis Oosthuizen (71) shared the fourth spot on two shots off the pace. After another poor major, he could feel the pain in his upper back muscles.
Asked how his body was feeling after the round, he replied: "Right now I can feel my left rhomboid [back muscle] going into spasm. It's sort of the way it has been the last few weeks", McIlroy said.
"I wanted to play the season".
McIlroy said he felt his "left rhomboid [back] going into spasm" after his final round at the US PGA, where he finished at one over for the tournament. The inside of my left arm goes numb. So I don't know what to do. "I have got this next week off to assess what I need to go forward".
McIlroy, 28, won in May a year ago at the Dubai Duty-Free Irish Open before claiming titles twice in three starts at the Deutsche Bank Championship and Tour Championship in September. I feel like I'm capable of playing well and winning and putting rounds together. April is a long way away.
McIlroy hasn't won a major since the 2014 PGA and struggled again this week.
"I was battling. I was scrambling well to be under par for that back nine". That is not where you want to be.
McIlroy said the pain is not almost as severe as in May at the Players Championship when it flared. It's there. I can feel it.
He then played two par effort before making another bogey on the par-three sixth hole. If I was injury-free, that wouldn't happen.
With the upcoming week already scheduled as an off week, McIlroy said he'll fly back to Northern Ireland on Sunday night and consult with doctors there.
"I just wanted to put my head where it needed to be which was trying to focus on the golf", he said.
"I've missed a lot of time already. But then at the same time, if you are not capable of playing at your best, why should you play?" he said. "But take him out of the equation, I feel like I'm still right there in the tournament". "It really depends", said the 28-year-old, who was out for seven weeks when he first sustained the injury, which he aggravated at The Players Championship in May and spent another month on the sidelines.
He returned with three top-10s in four starts in March and April, including a T-7 at the Masters, when he began experiencing pain in his back during The Players Championship in May.
McIlroy took time off to get married and enjoy his honeymoon, then quickly resumed full training.