The Merseyside outfit are unsure on the severity of the 20-year-old's injury and Roberto Martinez has admitted the England international could be out for up to five months
Everton midfielder Ross Barkley will be out for a "long, long time", according to manager Roberto Martinez.
The 20-year-old sustained medial ligament damage in training on Friday as Everton finalised preparations for their Premier League opener at Leicester City on Saturday.
And Martinez said he expects the 20-year-old to be sidelined for anything between seven weeks to five months.
"We had a scan but it was difficult to determine the extent of the injury. It is a problem with a medial ligament in his knee, but we don’t know how bad the injury is," Martinez said.
"We know there is a partial tear of the ligament and that could mean anything from seven weeks to four or five months, so we need to be cautious."
The Spaniard admitted the injury comes as a huge blow to him and his team, but insists the squad can cover for the loss.
"I think Ross Barkley is a phenomenal footballer and we are going to miss him – there are no two ways about it," he added.
"He was so sharp in pre-season and I think he came back with a different approach and a different mindset.
"He was very unfortunate to get injured in such an innocuous challenge in training and now he is going to be out for a long, long time.
"The squad should be big enough and be able to cope with losing a player, but it is a disappointing.
"Now we have to cope with that frustration and allow him to work on other aspects and make sure that when he comes back, he’s fit and ready for what is left in the season."