Brett Okamoto of ESPN first reported the news.
Williams (11-1-0) is undefeated thus far under the UFC banner, having defeated the duo of Abdul Razak Alhassan and Alex Morono. Williams has competed twice before in the UFC and his total fight time has been less than one minute. Williams has picked up six of 11 professional MMA wins by KO/TKO, with another four coming by decision.
Pereira (24-11-0-2NC) has won just two of four in the UFC, with those wins coming over the duo of Danny Roberts and Zelim Imadaev. Pereira was set to challenge Burenzorig Batmunkh for the Heat welterweight title at Heat 43 in September of 2018, but Pereira missed weight for the fight. Pereira has competed 37 times as a professional MMA fighter and 14 of those bouts went the distance.
UFC Vegas 17 takes place on Saturday, December 19 from the UFC Apex in Las Vegas, Nevada with Leon Edwards and Khamzat Chimaev headlining. Fightful is providing live coverage of the event, which airs live on ESPN 2.