McIlroy has played just a handful of tournaments this year after battling through a rib injury, which he said was not a factor in him missing the cut at Erin Hills after shooting an ugly first-round 78.
"In a perfect world last week wouldn't have been my first week back," the Northern Irishman told reporters on Wednesday.
"It's the US 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 No 3 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.
