Everton manager Ronald Koeman says midfielder Ross Barkley "needs more tests" to assess the extent of his suspected hamstring injury.
The England international, who has been linked with a move to Tottenham, has not trained for several days but Koeman did not put a timeframe on his return.
The 23-year-old has one year left on his contract at Goodison Park but has failed to sign a new deal.
"He stopped training last Monday, it looks like the hamstring," Koeman said.
"But we need to do more tests today and tomorrow to know exactly what the injury is."
Speaking ahead of their Europa League qualifier with Hajduk Split on Thursday, Koeman added there was "no update" on speculation regarding Barkley's future.
Asked if the midfielder was in his plans this season, Koeman added: "I will wait until the end of August."
Koeman said last week there had been no offers for the 23-year-old and has not ruled out him staying at the club.
Meanwhile the club are set to announce the signing of Swansea City midfielder Gylfi Sigurdsson for a reported £45m.
The 27-year old passed a medical at the club earlier on Wednesday, Koeman said at the news conference.
"I have seen him in blue but I don't know if all the paperwork is done," he added.