Four-time major champion Koepka, 32, this week switched from the PGA Tour to the Saudi Arabia-backed enterprise, following the likes of Phil Mickelson, Dustin Johnson and Bryson DeChambeau. Koepka’s brother, Chase, also played LIV’s opening event at Centurion Club earlier this month.
Koepka, the current world No 19, will debut in Portland, Oregon, next week where he will compete in the series' second 54-hole, 48-man tournament for a share of $25 million in prize money. As a result, he will be suspended from competing on the PGA Tour.
Speaking late on Wednesday ahead of this week’s Travellers’ Championship, McIlroy, a staunch advocate of the PGA Tour, was asked if he was surprised by the American's decision.
“Am I surprised? Yes, because of what he said previously,” the world No 2 said. “I think that’s why I’m surprised at a lot of these guys because they say one thing and then they do another and I don’t understand.
“I don’t know if that’s for legal reasons or if they can’t - I have no idea - but it’s pretty duplicitous on their part to say one thing and then do another thing. In public and in private.”
At last week’s US Open, Koepka grew irritated at a line of questioning that focused on LIV Golf, complaining that reporters were casting a “black cloud” over the season’s third major.
LIV Golf confirmed on Wednesday that Koepka had switched to their tour, with his first appearance to come at Pumpkin Ridge Golf Club from June 30-July 2.
“The addition of Brooks is yet another example of the incredible fields LIV Golf is assembling as we build momentum in our first season and look towards the future,” LIV Golf CEO Greg Norman said in a statement.
“There’s no understating the impact that Brooks Koepka has had on the game of golf in the last five years. He carries a championship pedigree and record of success as one of the most elite players in the world.”
In response to Koepka’s departure, PGA Tour commissioner Jay Monahan said: “I am disappointed that Brooks Koepka has left and has joined the LIV Golf series.
“I have not had a chance to talk to him about it, and I look forward to having a conversation with him, but I am not going to share any more than that at this point in time.
“Every player that has left, I have great admiration and respect for the contributions they have made to the PGA Tour. And I think a lot of players have had a hard time making that decision.”
Monahan, who on Tuesday hosted a meeting with PGA Tour members, announced a number of changes planned for the game's lead professional circuit, including a combined increased purse size of almost $60m at eight tournaments next year.
Monahan also confirmed a new series of up to three international events – it is understood Europe, Asia and the Middle East have been earmarked to host - to be played after the conclusion of the autumn schedule for the top 50 players in the FedEx Cup standings.
Monahan said: “My read on the situation is that this environment is unsettling and the players want to make certain that their tour is doing everything it possibly can to grow and evolve in the manner that has got us to this point.
“We need to continue to be the ultimate platform for the best players in the world, a place they want to compete to prepare for the greatest championships in our game. The best competing against the best at historic venues with traditions and meanings and purpose.
“We are going down our path, and we are excited about what we have announced today and there is more exciting news to come. We are going to do it as a tour, as a collective, and a group of members that are absolutely behind their tour.”