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 said: "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."
Everton are also still to assess whether Séamus Coleman, Sylvain Distin and Steven Pienaar will be fit to travel for tomorrow's Europa League group game at FK Krasnodar.
Hughes happy with Moses
Stoke boss Mark Hughes hopes "man on a mission" Victor Moses can keep up the impressive form the winger has shown since joining the Potters on loan from Chelsea.
The 23-year-old Nigeria international provided the cross from which Peter Crouch headed in the 15th-minute winner, and caused constant problems for the visitors.
Hughes, whose side moved up from 17th to 11th in the table with the victory, said of Moses: "He's been exceptional since he came to the club.
"We are really delighted with what he has produced."