Everton manager Roberto Martinez says an injury to Kevin Mirallas could be more serious than first thought.
The 26-year-old Belgium forward came off with a hamstring problem during Saturday's 1-1 draw with Liverpool.
The club are still awaiting a full diagnosis on the injury before Thursday's Europa League clash with Russian side FK Krasnodar.
"It isn't a straightforward diagnosis but obviously the signs are not great," said Martinez.
"We had a few scans and we are going to make sure we are clear with the problem. It's not normal when he feels something like that and has to come off in the game.
"We know then that it is something serious.
"He has always been a very fit boy who is always ahead of schedule and we hope this time it is going to be exactly the same."
Everton are still to assess injuries to defenders Seamus Coleman and Sylvain Distin and midfielder Steven Pienaar before their second game in this season's Europa League.
The Toffees are top of Group H after beating German side VFL Wolfsburg 4-1 in their first match, while FK Krasnodar drew 1-1 away at French team Lille.