McIlroy followed an opening 72 with a promising 67 at Mount Juliet, but carded rounds of 73 and 74 over the weekend to finish two under par in a tie for 59th.
The four-time major winner will now head to the Renaissance Club for the Scottish Open before the final major of the year, the 149th Open Championship at Royal St George’s, the following week.
“I certainly don’t feel as good about where I’m at compared to where I was after the US Open,” said McIlroy, who held a share of the lead in the final round at Torrey Pines before finishing in a tie for seventh.
“But I’ve got another week to get ready and prepare and I obviously want to play well next week as well and have a chance to win. There’s nothing better than preparing for a major championship by getting into contention the week before.
“But I need to work pretty hard on my game the next few days before heading into the Scottish Open.
“I didn’t drive the ball well at all. I didn’t hit a fairway with my driver. The rest of the game actually feels OK, but when you’re not putting the ball in the fairway it’s hard to score.
“I need to hit the ball in play and I think it’s very much against how I play. I might just need to throw it back a bit and hit more irons off tees and get the ball in play, because that’s what’s going to help me at least avoid the big numbers.
“In some ways it’s nice going to two links courses the next two weeks because there isn’t as much of an emphasis on hitting driver as a course like here, so sort of looking forward to that.
“I feel like I’ve got a pretty good two iron that I can hit a lot the next couple of weeks and at least get the ball in play, because I feel like when I get the ball in play, I can actually play from there.”
In contrast to McIlroy, reigning Open champion Shane Lowry will head into his title defence on a high following a closing 66.
Lowry, who will spend the next week practising on links courses around Dublin, said: “That’s probably why I’ve been so frustrated over the first few days because I feel like I have that round in me.
“It was an easy six under today. Nothing really special. It was one of those days where it probably could have been better.
“It was my goal to go out and shoot a good number, hitting some nice shots and being aggressive and making a lot of birdies. It is a nice round to take into my week off now and get ready for the Open.”