Everton fear midfielder Ross Barkley could face an extended lay-off after injuring his hamstring in training on Monday.
The 23-year-old, who is the subject of transfer interest from Tottenham, Chelsea and Arsenal, had only just returned to training last week following a hernia operation.
Teammates saw him go down in pain clutching his leg during the session at Finch Farm and the England international had to be helped from the field.
He is due to undergo a scan to determine the full extent of his injury.
Barkley has one year left on contract but Everton say they have yet to receive a firm bid amid concerns suitors will offer around £25million later this month in an attempt to put the Goodison club under pressure to sell.
Manager Ronald Koeman warned last week that Everton would not allow Barkley to leave below their valuation, which was understood to be £50million at the start of the summer.
‘We won’t let him go cheap, of course not,’ said Koeman, who has seen Swansea City hold out for £50m for Gylfi Sigurdsson.
‘We have the experience of what we need to pay for players who came here and had one year of contract. If there is a club thinking we can wait until the last day and we put some money on the table and we get Ross Barkley, sorry, you do not get Ross Barkley.’
He added: ‘If there is an offer, maybe the club will not accept. He is a really good player; a young player, an English player. With all the valuations we have seen the last few weeks about players, then if you want to buy Ross it is a big [financial] deal.’