"In a perfect world last week wouldn't have been my first week back," the Northern Irishman told reporters on Wednesday.
"It's the U.S. Open, it's a high stress, high pressure sort of event. I hit my stride a little too late. I played well my last nine holes on Friday. I was just trying to shake the rust off before that."
The world number three was inaccurate off the tee and too often found the course's long fescue, which he had criticised event organisers for cutting back before the tournament's opening day.
McIlroy, however, was far from the only big name to miss the cut at Erin Hills.
World number one Dustin Johnson and number four Jason Day also failed to make it into the weekend play.
McIlroy hopes he can get momentum going in his debut at the par-70 Connecticut course.
"Mentally, I was not as sharp as I needed to be and that just comes from playing rounds of golf," he said.
"Hopefully, I can get on a good run over these next few weeks and hopefully this is the start of that run."
(Reporting by Rory Carroll, editing by Ed Osmond)
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