Sixteen months on, he's won two Majors and finds himself ranked No.1 in the world.
More than that, he's made a convincing case that we are watching a young man on a path towards golfing greatness.
McIlroy's dominant triumph at the 94th US PGA Championship was a performance beyond his 23 years. The Northern Irishman took a three-shot lead into the final round and never once looked likely to lose it. He missed some fairways and put himself in some difficult spots, but McIlroy's extraordinary talent always found a way. Moreover, his nerve was as steely as a veteran champion.
By the time he reached the 18th green, McIlroy had left the field in his wake and extended his lead to seven shots. His cap came off and the gathered crowds stood as one to give him a deserved standing ovation. Another Major; another Sunday romp to victory - it was like his US Open win at Congressional all over again.
This time he finished with a flourish. With his father watching on, McIlroy rolled in a 15-footer for birdie and the crowd erupted once more. His eight-shot victory at Kiawah Island in South Carolina matched the margin he won his first Major by, but this was arguably even more impressive.
McIlroy's final round of 66 blew the competition out of the alligator-infested waters. Ian Poulter's early birdie blitz threatened a challenge and there were others ready to pounce, but without the merest hint of encouragement from McIlroy, they had nothing to cling to.
It wasn't Poulter's fault he faded. And it wasn't a failing on the part of the field that nobody could catch McIlroy on Sunday. The man from Holywood was simply accessing a level of talent and application beyond that available to anybody around him. To that end, it was a performance you might well call Tiger-esque - albeit referring to the man we used to know, at the peak of his powers.
The one we have now continues to scratch around for his greatness.
Weekend rounds of 74 and 72 were filled with all-too familiar putting woes and Woods's challenge evaporated before it ever really boiled up.
But this US PGA Championship wasn't about Woods. It was about the man best placed to inherit his mantle, and if that does motivate the man with 14 Majors to keep striving then I don't know what will.
McIlroy is now a multiple Major winner. He's also the youngest player to win two Majors, beating the marks of both Woods and Jack Nicklaus.
From here, almost anything is possible.
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