It was the longest final day in 64 years at the PGA Championship because of rain, with Walker playing 36 holes on Sunday.
The American began the day sharing the lead with Japan's Hideki Matsuyama at the Quail Hollow Club in Charlotte, North Carolina. The one plus was off the course, as McIlroy got married in April.
"These guys going out this afternoon, they break 70, they have done a hell of a job", McIlroy said.
He started the day just two shots off the lead, and ended it in a tie for 16th, seven shots off leader Kevin Kisner.
"Right now, I can feel my left rhomboid going into spasm", said McIlroy after his fourth round.
The Holywood star produced a Seve-like par save at his opening hole - deliberately rocketing a low six-iron off a cart path, through a bunker and onto the edge of the 10th green after his approach had careered down the same cart path, 80 yards right of the green. "I'll play tomorrow and assess my options for the rest of the year", he said. "That could have been a six or a seven quite easily".
"But once I get done, having to go through the whole routine of getting it ready to go again the next day, you shouldn't have to do that". "I was pretty consumed with what the heck I was trying it do on the second shot".
McIlroy has already taken time off due to rib issues twice this year and explained Sunday that he's recently been feeling discomfort in his left rhomboid (a skeletal muscle that is found on each side of the upper back). McIlroy says the only tournament that really means anything to him is in April - the Masters, the only major he hasn't won.
While he missed the putt and signed for a two under 69 that leaves him nine shots behind leaders Kisner and Matsuyama on one-over par, he believes he has the game and the confidence to challenge over the weekend.
Along with failing to win a major for the third straight year, McIlroy hasn't won anywhere around the world since he won the Tour Championship last September to capture the FedEx Cup and its $10 million prize.
If this is it for McIlroy in 2017, it's certainly been a disappointing one on the golf course.
Kisner carded 7-under through 54 holes. Two birdies (12th and 18th holes) helped him finish the day at 72 which takes his 36-hole total to three-over 145. Thomas has four birdies in his third round as he seeks to win his first major championship as well.
"Look, I don't know what I'm going to do", he said. "That aggravated it a little bit", McIlroy said.
Scott also gave a lukewarm thumbs-up to the hole locations, which some players thought were on the edge of being unfair on Saturday. Not quite where it has been the last month or so, but that's been phenomenal. So I don't know what to do.