England and Everton midfielder Ross Barkley could be ruled out for up to five months with a knee injury picked up in training this week.
The 21-year-old damaged the medial ligament in his right knee just a day before Everton's Premier League opener against Leicester City, which ended in a 2-2 draw.
Roberto Martinez has revealed that they will have to wait to find out the full extent of the damage, but the Toffees boss knows he could be without one of his key players for a large chunk of the season.
Barkley impressed for England during the summer's World Cup
"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," he revealed.
"We know it's a partial tear but that could be anything between seven weeks and I've seen injuries like that 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 to [sustain the injury] through an innocuous challenge in training and now he's going to be out for a long, 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.
"Now it's coping with the frustration and allowing him to come back when he's fully fit and ready for whatever's left in the season."
Barkley was part of Roy Hodgson's England squad for this summer's World Cup in Brazil and is now likely to miss September's games with Norway at Wembley and the Euro 2016 qualifier against Switzerland.
