The UFC 303 pay-per-view bout was set to be McGregor's first octagon appearance since July 10, 2021. The bout ended with McGregor suffering a gruesome leg fracture after a kick from Dustin Poirier.
Chandler, meanwhile, has not fought since November 2022, when he suffered a submission loss to Poirier.
With McGregor vs. Chandler no longer on the card, light heavyweight champion Alex Pereira will defend his title against Jiri Prochazka on short notice in the new main event.
Pereira and Prochazka previously fought at UFC 295 on November 11, 2023, with Pereira earning a second-round TKO win to claim the vacant light heavyweight title.
UFC 303 takes place on Saturday, June 29, at T-Mobile Arena in Las Vegas, Nevada. The event is part of the promotion's International Fight Week.