You have to go back two years to Zach Johnson's British Open triumph at Saint Andrews, which came eight years after his 2007 Masters triumph.
Since then Jason Day, Danny Willett, Dustin Johnson, Henrik Stenson, Jimmy Walker and Sergio Garcia have all proved one-hit wonders - so far - with Koepka seeking to avoid their fate.
Unlike tennis, where Roger Federer bagged his eighth Wimbledon title at the age of 35, golf has seen a constant changing of the guard. Four-time major winner McIlroy, who last triumphed at the 2014 US PGA, predicts the era of the dominant force is over.
"Yeah, I agree with Rory," said Koepka, whose only other PGA Tour win was at the 2015 Phoenix Open. "You look at how many good players there are. You look at how it was at Erin Hills.
"Everyone up there hadn't won a major. Rickie (Fowler), Justin (Thomas), Hideki (Matsuyama), they haven't won majors, and I think everyone in this room knows they're going to win one. It's only a question of when, not if."
But the question for Koepka is can he do it again. At least the 27-year-old probably stands a better chance of winning than many of his compatriots because of his familiarity with links golf in Britain.
He spent his formative years on the Challenge Tour, the feeder circuit for the European Tour. When Zach Johnson won at Saint Andrews, Koepka produced a final-round 68 for a creditable joint 10th,
"I love links golf. I think it's the best kind of golf," he said. "So much imagination that goes into it. You can play ten different shots from the middle of the fairway and I think that's so cool. You've really got to be creative."
The Royal Birkdale course is set up to penalise the wild but reward the persistent, with light rough offering a chance to those who stray marginally off the fairways.
"Playing over here for two years, you play a lot of links golf and you learn to love it and I have."
Koepka has enjoyed an astonishing progression since the 2013 British Open at Muirfield, which he reached after a 300-mile dash down from Scotland for the Sunningdale qualifying tournament, producing rounds of 69 and 65 after just two hours’ sleep.
After two years scrambling to secure a future, successive steps up to the European and then PGA tours laid the foundations for his U.S. Open win.
As if to remind him of that success, he will play with Matsuyama, runner up at Erin Hills, for the first two days, a pairing that will at least remind him of what is possible.
"Anytime you play with Hideki, you know he's going to play well. Sometimes you can kind of feed off that and hopefully it does this week."
Koepka has taken an extended break since the U.S. Open but fully intends to resume where he left off.
"I'm refreshed. I planned to take some time off and kind of regroup. But last week after July 4th it was kind of like, okay, I just want to get back out here, I want to play, I want to get inside the ropes. I'm really getting antsy."
(Reporting by Neil Robinson, editing by Larry King)
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