The six-time major winner, who succesfully defended his Masters title earlier this year, has played a limited schedule since, with a couple of tied-seventh finishes at the PGA Championship and Scottish Open his best showings and only top-10s in seven tournaments.
McIlroy, who went into the first of three FedExCup Playoffs 12th in the standings, is bidding to win a record-extending fourth seasong-ending title, but posted rounds of 74, 70, 72 and 72 to finish eight over.
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"Obviously, I didn't come in here with a ton of reps," McIlroy said. "I felt like the more the week went on, the more I felt lost with my golf swing.
"Figured it out a little bit the last few holes today but my swing wasn't in sync and I didn't do a ton of practice coming in here."
The top 50 in the standings after the FedEx St. Jude Championship progress to the BMW Championship, with the top 30 then qualifying for the season-ending Tour Championship.
But McIlroy feels like he is a long way from being competitive, with his game needing a lot of work before next week's BMW Championship.
He added: "I'm going to have to work pretty hard, put my head down and grind between now and next Thursday to have any sort of golf game to compete next week.
"I'll go to Bellerive Country Club and try to prepare for that golf course. But it's more about the execution about what I'm doing right now and that wasn't there this week.
"I am so far away from where I need to be to even feel like I have a chance so I've just got to keep working and hope it turns around a little bit."
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