There are fears the 20-year-old could be out for several months after suffering the injury in training last week, with swelling around the knee preventing an accurate prognosis.

"I don't know where the figure of five months came from but obviously the extent of the injury is a little unsure at the moment because we don't know how the next two weeks are going to develop," Martinez told Everton's website on Thursday.

"It's a medial ligament injury and we know that Ross, in terms of the power and the strength that he has in his body, he could easily react to treatment a lot better than other players.

"The length that Ross will be out is a little uncertain at the moment and it could be anything between the eight weeks that we mentioned or a little further.

"Until the end of the week, or probably the next 10 days, we won't know how the knee is progressing and how quickly Ross can be back."

Barkley suffered a broken leg at the age of 16 but recovered to establish himself in the Everton side and then earn England selection, with his major breakthrough coming last season when he scored six goals and made 34 league appearances.

He was a member of England's squad at the World Cup in Brazil and signed a new four-year contract at Goodison Park last month.

"Obviously we are keeping an eye, very closely, on how that knee reacts and every time we have a real improvement we are checking that," Martinez said.

"At the moment his knee is in a brace and this is a very important period of rehabilitation, making sure that the ligament can be repaired in a good position."