Five-under through 10 holes of his second round, defending UBS Hong Kong Open champion McIlroy simply hit the wall, dropping six shots in his final seven holes to miss the cut by three.
The effervescent golf was played instead by Kiwi Campbell (43), who leapt into a one-stroke lead with a superb 64, then revealed how close he'd come to quitting the sport in the grim years that followed his 2005 US Open victory at Pinehurst.
He explained: "Late one night in Sydney, when I was back at home with my family, I said to myself 'okay, tonight is where I decide whether I'm going to give up the game or keep on playing'.
"I went to bed and when I awoke in the morning, something inside me said 'keep on going, you've still got the talent'," Campbell added. "It doesn't go away, the talent. It's always with you. It just hides now and then."
Winning had seemed so easy in the summer of 2005, when Campbell came straight from Pinehurst to the K Club and romped to victory at the European Open, yet he'd soon lose his game, his confidence and all but the last vestiges of hope.
Seeing the feats of fellow fortysomethings like Thomas Bjorn, Paul Lawrie and then Ernie Els at this year's British Open, nurtured Campbell's self-belief, and he finally got a hint of redemption at last month's third-place finish at the Portugal Masters.
Campbell smiled mischievously when told McIlroy missed the cut, saying: "that makes life a lot easier ... Rory's world No 1 for a reason. He's a wonderful talent and what's he's done for the game and for himself is fantastic. Yet the fact he's not here this weekend definitely makes winning a lot easier!"
After wrapping-up the Race to Dubai title in Singapore, the edge clearly had gone off McIlroy's game during Thursday's two-over 73, but he started the second round with such a flourish, the Holywood star began to get the faintest whiff of victory in his nostrils.
Yet his adrenalin simply drained away when he made a slipshod par-five from the middle of the fairway on Fanling's third hole, his 12th. "That was a real momentum-killer," admitted McIlroy, whose putter suddenly started to sputter.
He then followed a couple of three-putts with a careless four-stab double-bogey at the 10th, his last, after all hope of making the weekend had evaporated.
In all, McIlroy took 70 putts on Thursday and yesterday as he missed his first cut since June's US Open and his fourth this year. However, he looked forward to a mixture of rest and hard work on his putting in the run-up to Thursday's DP World Championship in Dubai.
Peter Lawrie, who started this tournament in the final qualifying spot for Dubai, showed laudable strength of character to post two rounds of 68. Tied eighth on four-under, the Dubliner's playing well enough to clinch the top-12 finish which would almost certainly clinch his ticket to the European Tour's Grand Finale.
A wanderly Padraig Harrington played out of the 'flyer rough' far too often to expect any better than the 75 which sealed his fate on six-over. Muskerry's Niall Turner shared 88th place with McIlroy after his second-round 71.
Hong Kong Open,
Live, Sky Sports 2, 5am