Jose Mourinho has suffered another major setback after it emerged keeper Thibaut Courtois is expected to be out for at least two months after suffering a serious knee injury.

Chelsea doctors were attempting to assess the full extent of the issue and although substantial swelling was preventing them from making an exact diagnosis, Courtois has already been warned he faces an extended spell on the sidelines.

Chelsea’s initial fears that Courtois had suffered cruciate-ligament damage are believed to have been dismissed, but the 23-year-old Belgian international has been ruled out of Saturday's trip to Everton.

Thibaut Courtois is expected to be ruled out for at least two months with a serious knee injury

Chelsea have suffered another setback after their stuttering start to the defence of their Premier League title

Mourinho is under mounting pressure at Stamford Bridge after just one win in the opening four Barclays Premier League games of the season and the loss of his first-choice keeper is another major blow.

Courtois has established himself as one of the world’s best goalkeepers since his return to Chelsea from a loan spell at Atletico Madrid.

Although he has had a difficult start to the season following his dismissal in the opening-day draw with Swansea, Mourinho needed his main man at Goodison Park. Summer signing Asmir Begovic will deputise.

Chelsea have a tricky run of fixtures coming up and next week turn their attentions to Champions League action with a group game at home to Maccabi Tel-Aviv.

Courtois conceded two goals against Crystal palace as the defending champions slipped to a second defeat

New signing Asmir Begovic (left) will deputise for Courtois while is recovers from injury

After his return to the Chelsea team following suspension, Courtois conceded twice in the victory at West Brom and two more in the defeat at home by Crystal Palace before the international break.

He started Belgium’s European Championship qualifying victories against Bosnia-Herzegovina and Cyprus before returning to Chelsea on Tuesday.

The keeper trained on Wednesday, but suffered the injury during practice which immediately ruled him out of the Everton trip.

Another player facing a lengthy injury absence is Arsenal’s Jack Wilshere. He could be out for another three months after making a slow recovery from a hairline fracture to his left leg.

The Arsenal midfielder is not expected to be fit for England’s next round of Euro 2016 qualifiers in October and surgery has not yet been ruled out.

It is another blow for Wilshere, 23, who was hurt in training four weeks ago. He only returned in May after five months out with an ankle injury.

Jack Wilshere's injury woes also look set to continue with the Arsenal star likely to be out for three months

Manager Arsene Wenger had hoped he would be back in time to face Stoke City at the Emirates on Saturday but met with Arsenal doctors to discuss his condition. He could be back in four weeks but if surgery is required, the recovery period may be up to three months.

‘I have not seen Jack Wilshere in the last two or three days,’ Wenger said. ‘He is progressing more slowly than expected.

‘I expected him to be back and available after the international break, but unfortunately he will not be available for Saturday. He had a little setback.’

Wenger, who confirmed that Danny Welbeck will be out until Christmas, also responded to fans’ criticism of Arsenal’s failure to sign a striker in the transfer window and called for unity.