Defender Masina and creative midfielder Pereyra missed Pearson's first game as Hornets boss – a 2-0 defeat at leaders Liverpool – because of minor injury problems.
Left-back Jose Holebas (ankle) remains sidelined, along with long-term absentees Tom Cleverley (heel), Danny Welbeck (hamstring), Sebastian Prodl, and Daryl Janmaat (both knee), while new signing Joao Pedro is ineligible to make his debut until January.
Paul Pogba remains absent for United.
The 26-year-old has only made six appearances this season due to an ankle injury but Ole Gunnar Solskjaer has not given up on the midfielder returning before the end of 2019.
Diogo Dalot will play for United's youth side against their AC Milan counterparts on Friday as the full-back builds fitness, while Eric Bailly and Timothy Fosu-Mensah were due to start full training this week. Marcos Rojo is also continuing to build his fitness.
Watford provisional squad: Foster, Mariappa, Cathcart, Kiko, Kabasele, Doucoure, Hughes, Capoue, Deulofeu, Deeney, Sarr, Gomes, Dawson, Masina, Foulquier, Chalobah, Pereyra, Quina, Gray, Success.
Manchester United provisional squad: De Gea, Romero, Grant, Kovar, Jones, Laird, Lindelof, Maguire, Shaw, Wan-Bissaka, Tuanzebe, Williams, Young, Fred, Garner, Gomes, James, Mata, Matic, McTominay, Pereira, Lingard, Chong, Greenwood, Martial, Rashford.