The LIV series concludes at Doral, near Miami, this weekend with a team championship featuring eye-watering multi-millions as prize money but the animosity the breakaway has caused has left McIlroy worried for the game.
"This 'us versus them' thing has gotten way out of control already," McIlroy said in an interview with British newspaper The Guardian on Wednesday.
"If the two entities keep doubling down in both directions, it is only going to become irreparable. We are going to have a fractured sport for a long time. That is no good for anyone," said the Northern Irishman, who returned to the top of golf's world rankings with victory in the CJ Cup on Sunday.
Mickelson, speaking at the press conference for the LIV event, said he did not want to distract from the tournament at the Blue Monster course by responding directly to McIlroy's comments but he said the breakaway was now part of the golf scene.
"If I'm just looking at LIV Golf and where we are today to where we were six, seven months ago and people are saying this is dead in the water, and we're past that, and here we are today, a force in the game that's not going away," he said.
Sitting on a platform with British Open winner Cameron Smith (29), former Masters winner Bubba Watson (43) and English Ryder Cup favourite Ian Poulter (46), Mickelson said the quality of talent in the LIV series was "moving professional golf throughout the world and (creating an) excitement level in the countries around the world”.
"It's pretty remarkable how far LIV Golf has come in the last six, seven months. I don't think anybody can disagree with that," said the American.
While McIlroy talked of the danger of polarisation in the sport, he also took aim at former team-mates from the Europe Ryder Cup team who had signed up for LIV.
"It's a weird thing. I think it is the first time in my life that I have felt betrayal, in a way. It's an unfamiliar feeling to me. You build bonds with these people through Ryder Cups and other things. Them knowing that what they are about to do is going to jeopardise them from being a part of that ever again?” he said.
But one of those team-mates, Poulter, objected to the terminology used by McIlroy.
“A betrayal? We can still qualify for the team as far as I'm aware. Unless we've been told we can't qualify, then I'm still ready to play as much as I possibly can and try and make that team," he said.
"My commitment to the Ryder Cup I think goes before me. I don't think that should ever come in question. I've always wanted to play Ryder Cups and have played with as much passion as anyone else that I've ever seen play a Ryder Cup.
"You know, I don't know where that comment really has come from, to be honest,” he said.