The Brazil forward's injury and subsequent absence were inadvertently revealed after he missed a Christmas lights switch-on event in Altrincham.
Although the injury is not believed to be serious, it presents a further selection headache for manager Ruben Amorim.
Other key players, Benjamin Sesko and Rasmus Hojlund, are also unavailable due to injury and being out on loan, respectively.
Joshua Zirkzee will make his first Premier League start of the season, supported by Bryan Mbeumo and Amad Diallo, while Leny Yoro replaces the injured Harry Maguire.