"You cannot play this golf course if you're not in position off the tee, and I wasn't in position," he said.
"Obviously, I paid the price for it today."
Before the tournament began, four-times major winner McIlroy, one of the game's best drivers of the ball, had spoken about how eager he was to attack the long, links-styled, rolling farmland course off the tee.
McIlroy has played just six tournaments this year after battling through a rib injury but said pain was not to blame for his performance on Thursday.
Rust was more of an issue, he added.
"I was hacking around in the rough out there and didn't feel it one bit. So, no, that's totally fine," he said.
"I started missing some left on the course and tried to sort of correct it on the way in and missed a couple right," he said.
"So a little bit of timing, and a bit of rust as well."
The 28-year-old acknowledged that it would be a steep climb to get back into the tournament after a day when 17 competitors broke 70, but he said he was up for the challenge.
"I watched some of the coverage earlier on and saw what Rickie (Fowler) did and Brooks Koepka and some of those guys," he said.
"I thought I was on course to do that after being 2-under through two, and didn't work out that way."
(Editing by Peter Rutherford)
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