Alvarez's coach, Mark Henry, mentioned the Dec. 2 date in a recent Instagram post.
Alvarez and Gaethje coached against one another on the 26th season of "The Ultimate Fighter" reality series. The TUF 26 finale will take place Dec. 1 in Las Vegas, but the two coaches will fight in the 155-pound bout the following day on the UFC 218 pay-per-view.
Alvarez (28-5) is looking for his first win since taking the lightweight championship with a first-round knockout of Rafael dos Anjos in July 2016. The Philadelphia native attempted to defend the title last November, but suffered a TKO loss to Conor McGregor.
In May, Alvarez fought Dustin Poirier to a no-contest. It was a back-and-forth affair that ended when Alvarez landed a series of illegal knees.
Gaethje (18-0) is a former World Series of Fighting champion. He made his UFC debut in July, knocking out Michael Johnson in the second round. Fighting out of Denver, Gaethje boasts 15 career knockouts.