Rory McIlroy, off to a slow start in the PGA Championship at Quail Hollow Club in Charlotte, reportedly is using a different driver this week after the one he utilised during his recent Masters win was ruled "no longer permissible" by the USGA.

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The report from SiriusXM PGA Tour Radio on Friday indicated that the USGA deemed McIlroy's driver non-conforming earlier this week after routine testing.

McIlroy has been less available to reporters than usual this week, declining to meet with the press following his 3-over-par 74 on Thursday and then again on Friday, despite his rally to make the cut with a 2-under 69.

Those scores reflect poor results with the new driver, as McIlroy hit just four fairways on Thursday and six on Friday.

His media unavailability has kept the nature of his non-conforming driver a mystery, and a myriad of factors could be responsible.

Drivers have been known to pass inspection at one venue and not at the next, per media reports. More extreme examples include drivers conforming at a manufacturer's test and then failing a USGA test a week later.

Damage to the driver can cause an issue, as can simple usage, which can gradually take a club from a place of conformance to something deemed unacceptable.

The Masters victory, giving McIlroy a career Grand Slam, coupled with his silence this week will raise further speculation, but he already has his defenders.

Fellow golfer Johnson Wagner provided additional context when questioned about it by SiriusXM.

"This is something that happens week to week on the PGA Tour," Wagner said. "It's unfortunate that it happened the week of a major. ... He did nothing wrong."