Gastelum withdrew due to an injury to his mouth, sources told ESPN. Middleweight contender Sean Strickland has agreed to replace Gastelum on the card and will face Imavov in a five-round, 205-pound main event.
Saturday's UFC Fight Night event will take place at the Apex in Las Vegas. UFC officials confirmed the switch in the main event.
Imavov (12-3) is looking to continue his move up the rankings, as he has won three in a row against Ian Heinisch, Edmen Shahbazyan and Joaquin Buckley. He is currently No. 12 in the UFC's official rankings.
For Strickland (25-5), who trains out of Las Vegas, this will mark his second consecutive headliner on the UFC's schedule. The UFC closed out 2022 with a middleweight main event between Strickland and Jared Cannonier. Strickland lost by decision.
ESPN's Carlos Contreras Legaspi contributed to this report.