Everton fear that forward Kevin Mirallas' hamstring injury will result in a lengthy absence, with manager Roberto Martinez describing the early diagnosis as "not great."
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The Belgium international was carried off on a stretcher after pulling up during the first half of Saturday's 1-1 draw with Liverpool at Anfield.
Martinez says it is too early to say how long Mirallas will be out for, but has admitted that he is concerned.
He told Everton's official website: "With these types of injuries, sometimes you need more than 48 hours for the inflammation to go down.
"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.
"We need to make sure we get the full report before we know exactly what is wrong with Kevin.
"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."
Mirallas, 26, had an injury-hit first season at Goodison Park after arriving from Greek side Olympiakos for 5.3 million pounds in August 2012.
He recovered to play a key role as the Toffees finished fifth last season, scoring eight times in 32 Premier League appearances before a groin problem forced him to miss the final three matches of the campaign.
Having helped Belgium reach the quarterfinals at the World Cup in Brazil, he had scored three goals in seven appearances for Everton this season before his injury.
Martinez said: "His game has gone from strength to strength and the way he had started the season had been really positive. What he did at the World Cup pushed him to the next level.
"Last season he had a very important role and this season he had started in the same manner. He was scoring goals and being a real threat in everything we do going forward.
"We are going to miss him, so that is why it is so important that the medical team work well with his rehab and make sure that when he comes back, he comes back refreshed and stronger.
"We have plenty games left for him to be as influential as he has already this season."