Everton boss Roberto Martinez has said he fears England midfielder Ross Barkley could be ruled out for months.
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Barkley, 20, damaged the medial ligament in his right knee in training the day before their opening Premier League match at Leicester, which ended in a 2-2 draw.
Everton will have to wait to find out the scale of the damage, but Martinez said: "He had a scan but it was a bit difficult to determine the extent. With the swelling, we don't really know how bad the injury is.
"We know it's a partial tear, but that could be seven weeks... and I've seen injuries like that take four or five months.
"Ross Barkley is a phenomenal footballer and we're going to miss him, there's no doubt about it. He was so sharp in pre-season -- I thought he came back with a different approach.
"He was very unfortunate, and now he's going to be out for a long time.
"The squad should be big enough to be able to cope with losing a player and be able to perform well, but it's a disappointing moment when Ross Barkley was going to have a really big start to the season."
The injury means Barkley looks certain to miss the Euro 2016 qualifiers against Switzerland, San Marino and Estonia.
But Martinez backed him to cope with the frustration, saying: "He's a very mature boy. He's been through periods of bad adversity before, and he knows how to cope with that.
"As a football club, we're going to be with him and make sure that he's not suffering too much while he's away."
The Goodison Park boss, whose side led twice at Leicester through Aiden McGeady and Steven Naismith before being pegged back, said he was disappointed not to have taken all three points, adding: "We should have won but it's only us to blame.
"I thought in the first half we did everything we had to do, we controlled the environment. We did the hardest thing, which is get yourself in front twice.
"The first goal we conceded was very unfortunate, just a rebound from a corner, and the second one we probably got too relaxed.
"If I've got a criticism from our point of view it's probably we should have gone for that third goal with a bit more brightness rather than just controlling the game."