Roberto Martinez says it is too early to tell how long Ross Barkley will spend on the sidelines with a medial knee ligament injury.
The midfielder suffered the setback in training and underwent a scan on Saturday to determine the severity of the problem.
The results have proved inconclusive but Martinez hopes the extent of the England international’s injury will become clearer in the next few days.
Martinez said: “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.
“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.”
Martinez admits it is a huge blow to lose a player of Barkley’s quality who had impressed the Blues boss during pre-season.
“I think Ross Barkley is a phenomenal footballer and we are going to miss him – there are no two ways about it,” Martinez said. “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.”