Everton manager Roberto Martinez fears forward Kevin Mirallas may have suffered a serious hamstring injury.
The Belgium international pulled up during the first half of Saturday's 1-1 Merseyside derby at Liverpool and left the field on a stretcher.
Everton are still to learn the full extent of the problem but Martinez admitted he is concerned.
The Toffees boss told evertontv: "We had a few scans and we are going to carry on today. It isn't a straightforward diagnosis but obviously the signs are not great.
"We are going to make sure we are clear with the problem. It's not normal when Kevin Mirallas 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 also still to assess whether Seamus Coleman, Sylvain Distin and Steven Pienaar will be fit to travel for Thursday's Europa League group game at FK Krasnodar.