The world No.2, a winner of the championship two years ago at Congressional, has been at odds with his game for the majority of the year and that pattern continued at Merion where he finished a distant 14-over-par.
Even so, the 24-year-old insisted afterwards that he feels like his game is gradually coming round.
"There are signs during every round that it's there and then it sort of hits a bit of a struggle for a few holes and then it comes back," said McIlroy, who had rounds of 73, 70, 75 and 76 at the Pennsylvanian course.
"It's just a bit of a struggle and it seems like if I've had a few too many tournaments this year where I've struggled in one round or a couple of rounds."
"I think it's about confidence and fluidity. I don't have any other way to explain it. I have been working on the swing quite hard over the last few months and it looks really good on camera. I just get on the course and I hit a couple of bad shots and I guess that it sort of makes you lose confidence a little bit during the round.
"It's just a bit of a struggle and it seems like if I've had a few too many tournaments this year where I've struggled in one round or a couple of rounds and not been able to put it all together.
"I don't feel like it's too far away at all. It's just a matter of believing and staying patient and working hard and knowing that if you work on the right thing, you are going to turn it around."
McIlroy will continue his ongoing search for the form that saw him top the money lists on both sides of the Atlantic last year at the Irish Open in just under a fortnight's time.