Barkley suffered medial ligament damage in his right knee during a training session just 24 hours before Everton kicked off their Premier League campaign with a 2-2 draw at Leicester City on Saturday.

It was initially believed Barkley would be ruled out for six to eight weeks.

But Everton are still waiting to find out the full extent of the damage and Martinez fears he will be without one of his key players for a large part of the season.

"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," Martinez said during his post-match press conference at the King Power Stadium.

"We know it's a partial tear but that could be anything between 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 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, but it's a disappointing moment when Ross Barkley was going to have a really big start to the season."

The injury is also a blow to England, with Barkley, who had featured for Roy Hodgson's team at the World Cup, looking certain to miss the Euro 2016 qualifiers against Switzerland, San Marino and Estonia.

However, Martinez backed the 20-year-old to cope with his latest setback, adding: "He was very disappointed because he was looking forward to the start of the season.

"He put a lot of extra work in because he came back from the World Cup with a real desire to start the season well.

"But he's been through periods of bad adversity before and he knows how to cope with that."