"JP did a great job," McIlroy later said, after recovering from five-over after six to record a one-over round. "He kept me positive out there and that was very much appreciated."
However, two days later, McIlroy's charge for a second Claret Jug was derailed, when he took a double bogey on the 10th, courtesy of the wrong club selection off the tee. Again, the spotlight picked out Fitzgerald.
In the caddyshack, the development was not greeted with too much surprise. Do not expect an overload of sympathy, either, and not just because Fitzgerald is estimated to have earned more than £8m ($NZ14m) in his employment with McIlroy.
"It was coming," one caddie told Telegraph Sport. Fitzgerald's meticulousness has been called into question by his peers, some of whom believe the mistake at Birkdale was merely the latest error.
Within the game, McIlroy has long been urged to employ a new caddie, but the famously strong-willed McIlroy remained fiercely loyal to Fitzgerald, who previously worked for the likes of Darren Clarke and Paul McGinley.
When American Jay Townsend, a former pro who is an analyst for various media outlets, including the BBC, criticised McIlroy's course management and pointed the finger at Fitzgerald, the golfer vociferously hit back on Twitter.
But, at last, the need to freshen up and, of course, to try to emulate Jordan Spieth's brilliant strategy skills, which were instrumental in him winning his third Major at Birkdale, look to have persuaded McIlroy that this was the moment for a change.
Yet the timing is curious to say the least and suggests that there had been no planning in this parting of the ways. Apart from the few months at the start of his professional career, McIlroy has only known Fitzgerald in his corner.
The 28-year-old's managers refused to comment on this issue, saying nothing more than he will answer questions on Wednesday at his pre-tournament press conference. That will be yet another colourful scenario in McIlroy's ever-turbulent story.
Diamond could even prove the permanent solution. A childhood friend, who grew up in the same Belfast suburb of Holywood, Diamond is a fine player in his own right, having played with McIlroy in the Ulster youth team and going on to represent the Irish senior team.
And he has caddied for McIlroy before. When the then 16-year-old prodigy was invited to play in the Irish Open in 2005, Diamond was on his bag.
The pair are extremely close, with Diamond reportedly acting as best man at McIlroy's wedding to Erica Stoll in April.
Tommy Fleetwood and Ian Finnis have shown over the last 12 months that best friends can work together as player and caddie - the duo has risen from outside the world's top 150 into the top 20 - but there could be a stumbling block in this instance, as Diamond is a successful businessman, owning and managing a number of pubs and clubs in Belfast.
Inevitably, there is speculation that Jim "Bones" Mackay, who has recently split with Phil Mickelson after 25 years together, will step up and Joe LaCava will be another popular suggestion, as he is Tiger Woods's caddie and has not worked since February because of his employer's continuing troubles.
Whatever and whoever, be sure that there will be no shortage of applicants.
The four favourites to replace Fitzgerald
Harry Diamond
McIlroy may well have looked at the success story of Tommy Fleetwood and best friend Ian Finnis, and decided that he and the former Irish international can work the same magic. However, Diamond is a successful businessman.
Joe LaCava
McIlroy is close to Tiger Woods and do not rule out the possibility of the latter telling his friend that he can employ his bagman. LaCava is rated as one of the best in the business, having worked for Fred Couples and Dustin Johnson.
Jim "Bones" Mackay
Phil Mickelson stunned the game in June when announcing that, after 25 years and five Major titles, he and Bones had split. Mackay has since signed a TV analyst contract, but would be an ideal fit for McIlroy. He was never scared to question Mickelson.
Jonathan Smart
Perhaps a left-field choice, but McIlroy and Danny Willett's former caddie are friendly, and "Smartie" is admired on the circuit. Another fine player in his own right, the Yorkshireman could be a wise pick in ensuring the focus remains on the golfer.
