Allan had to receive treatment on a muscle injury at Finch Farm on Thursday.
And while Everton manager Rafa Benitez is hopeful the midfielder will be passed fit to face Brentford on Sunday, he was unable to say for certain if the Brazilian would be in the squad.
Allan is understood to have picked up the muscle complaint in the defeat to Manchester City.
Everton have been down to the bare bones in midfield of late with Abdoulaye Doucoure, Andre Gomes and Tom Davies having missed games through injury.
READ MORE: 'It's complicated' - Rafa Benitez makes Jean-Philippe Gbamin admission after double Everton decision
READ MORE: Rafa Benitez responds to Everton young player claims and sends blunt Richarlison message
Losing Allan would compound the Blues' current injury misery which also sees Dominic Calvert-Lewin, Yerry Mina and Andre Gomes on the sidelines, while Mason Holgate and Richarlison are suspended for the Brentford clash.
Jean-Philippe Gbamin is fit but Benitez has said 'it's complicated' with the £25m recruit who spent most of the first two years of his Everton career out injured.
Gbamin was also replaced at half-time in his only league start this season and, in the games since, has seen Holgate, a defender, and teenager Tyler Onyango brought on ahead of him.
Doucoure has been given a chance of being cleared to play against Brentford but Benitez is keeping his fingers crossed that he doesn't gain a midfielder and then lose one at the same time.
Allan will continue to be assessed, with Benitez saying: "I hope so.
"He was having some treatment today but my feeling is he will be fine."
