McIlroy was furious after a four-over-par first round as his bid for back-to-back majors started in disappointing fashion.
He started his second round at Aronimink Golf Club in south-west Philadelphia eight shots off the clubhouse lead, but will enter 'moving day' five back from Americans Alex Smalley and Maverick McNealy after a bogey-free 67.
Only two players have shot lower over the opening two days – Chris Gotterup with a 65 and Ludvig Aberg a 66 - and it is a congested leaderboard with 15 players within two shots of the midway lead for only the third time in major history.
McIlroy labelled his open round "s***" and, asked to describe his second, replied: "Less s***."
He added: "At five back I do feel like I'm right in the tournament and that's really what I wanted to do today was to just get myself back in it.
"I think this afternoon I had a better understanding of how the course was playing. I probably went out there yesterday being a little too aggressive, thinking that guys were going to go lower than they were.
"Because I certainly didn't, in the practice rounds, see it playing as difficult as it has played."
It was a slow pace, as is common at major championships. McIlroy sat down on the 10th tee as his group waited to play. It took over five hours to complete his round.
"It seems like the first two days of major championship golf are always going to be like that," McIlroy added.
Aronimink has certainly proved a stiff test over the opening two days, biting back at pre-tournament claims that it would not challenge the world's best golfers.
World No.1 Scottie Scheffler, who battled to a second-round 71 to sit two off the lead, said the pin locations were "the hardest I have seen on tour".
McIlroy has mixed feelings over how the course has played, questioning the packed nature of the leaderboard.
"I think a bunched leaderboard like this, I think it's a sign of not a great setup," he said.
"I think when it's as bunched as it is … because it hasn't really enabled anyone to separate themselves.
"It's easy to make a ton of pars, hard to make birdies, and it feels like bogey's the worst score you're going to shoot on any one hole."
"I think the setup is fine, the golf course is good, the pins were tough.
"I've always felt like really good setups, it starts to spread the field a bit and not great setups sort of bring everyone together.
"Where they have put these hole locations, I feel like they have really tried to protect the course the first couple of days. So it seems like they have used up a lot of the really hard ones."
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