Everton hope to discover on Thursday the extent of the injury suffered by midfielder Allan during their Carabao Cup win over West Ham.
The £21m summer signing was substituted 69 minutes into Wednesday's 4-1 win with what looked like a groin injury.
The Toffees also lost fellow Brazilian Richarlison and defender Jonjoe Kenny to injury during the game.
Boss Carlo Ancelotti said Richarlison is likely to be fit for the weekend but Allan and Kenny were "more difficult".
"Richarlison, twisted ankle - not so bad," said the Italian. "Allan and Jonjoe, adductor muscle - we have to check tomorrow.
"Richarlison can recover for Saturday, Allan and Kenny more difficult."
Ancelotti's side cruised through to the last eight thanks in the main to a second hat-trick of the season from striker Dominic Calvert-Lewin. Richarlison scored their other goal.
Everton face Brighton at Goodison Park on Saturday looking to extend their impressive 100% winning start to the season.
It is the first time they have started a top-flight campaign with three successive victories since 1993-94.
"I'm really pleased for the start of the season but it is not a surprise, this squad has a lot of quality," added Ancelotti. "We have to wait and see what is going on in the future but the start has been good."
On the subject of Calvert-Lewin, who has scored eight goals in five appearances this season, he said: "He is doing a fantastic job - he is on fire. I don't know if he is the best [English striker] and I don't care - he is the best at Everton."
Richarlison will be a huge miss - analysis
Former Everton midfielder Leon Osman on BBC Radio 5 Live:
Considering how Everton have started the season so well you want to get through games without injuries - and certainly not to your most crucial players.
Allan has been terrific since he came in. I think Fabian Delph could cover the role Allan has been playing for a week or two - but Richarlison will be a huge miss. He is a huge part of what Everton do.
He has pace, guile and ability and he'll be a big miss for Carlo Ancelotti.
[Richarlison's] was an impact injury. Allan's injury looked like a pulled muscle but Richarlison appeared to twist his ankle. Only time will tell the significance of those injuries.
It's unfortunate that they might have to do without Richarlison for a couple of weeks. It will be interesting to see how Ancelotti tries to deal with that.