The four-time major victor has been far from his best over the past year as he struggled with the injury he picked up in the close season, and has admitted to rushing back before regaining full fitness.
World number four McIlroy completed a disappointing major season with a three-under par 68 in the final round of the PGA Championship at Quail Hollow, finishing well back on one-over 285 for 72 holes in the year's last major tournament.
McIlroy remains one of the brightest talents in golf, but he has also failed to win a major since he claimed two in 2014.
Winless this year and having battled back and rib injuries, McIlroy suggested his 2017 could be over, but the Northern Irishman is unsure what his immediate future holds.
"Right now I can feel my left rhomboid going into spasm. I wasn't keeping his score", said Kisner, who was in Day's group along with Hideki Matsuyama.
"If you had given me one over par standing on the first tee yesterday morning I probably would have taken it going into the weekend".
McIlroy came into the US PGA Championship as the pre-tournament favourite on the back of top-five finishes in his last two starts and a brilliant record at the host venue of Quail Hollow, including two wins and a course record of 61.
Those mistakes led to bogeys at the second, third, fifth and sixth before he rallied in true McIlroy style, converting five footers for birdie at the par-five seventh and short, par-four eighth before leaving a 12 footer hanging on the left lip at the last. "That's the next big thing on my radar".
April is the Masters, the only major keeping McIlroy from the career Grand Slam.
We'll know more in a few weeks, and you know McIlroy wants to defend his FedEx Cup win, but like he noted, we might have already seen his last shot of the season.
"You know, I think today, the No. 1 thing for me was, I didn't really hit a lot of great shots but I didn't really hit a lot of really bad shots", Stroud said.
He finished the day with three birdies, four bogeys and two double bogeys but what hurt even more was the small tap- ins that did not fall and the chips that were way below his standards.
Bemused by the ups and downs of the game, Lowry said: "This game puzzles me every day of the week".
Spieth won't have another chance at major No. 4 until heading down Magnolia Lane in April, but he still believes there's plenty to play for as the season reaches what he termed the "fourth quarter".
McIlroy said the pain wasn't as bad as it was early in the year at the Players, when he said it really flared up on him.
"There are 36 holes to be played", said Lowry, who is ranked second this week for strokes gained putting. I got back and playing it felt OK through the Masters. He wants time off to make sure it is fully healed.
"Just sort of have a chat with him about it and see what we need to do going forward, " he said. If I was injury-free, that wouldn't happen.