One half of the biggest bromances in golf, friends had widely tipped McIlroy to be the one who would stand by McDowell as his best man on his big day.
However, McIlroy hit the town with Manchester United striker Rooney on the same night that McDowell was toasting his new wife with a small party of less than 30 people.
McIlroy cited work commitments in the UK as forcing him to miss out on his best pal's nuptials.
"Rory was in the UK to do a photoshoot for one of his sponsors Nike," a spokesperson for the golfer told the Herald. "Wayne Rooney is another Nike athlete and they did the shoot together."
McIlroy looked relaxed as he stepped out in Manchester without girlfriend Caroline Wozniacki who is currently in Japan.
It has been a busy time on the social scene for the Down native of late after he enjoyed two trips to Dublin in one week.