Socceroo Tim Cahill is a doubtful starter for the World Cup in Germany after it was revealed the Everton midfielder's knee injury was worse than first feared.

Scans have shown the 26-year-old faces six weeks on the sideline after sustaining a partial tear of the posterior cruciate ligament in his left knee after a collision with Birmingham's Damien Johnson in an English Premier League clash on Saturday.

With Australia's first match of the World Cup against Japan on June 12, just over six weeks away, Cahill's participation is in major doubt.

"Tim saw the consultant yesterday and he confirmed our diagnosis that he has suffered a partial tear of the posterior cruciate ligament," Everton head physio Mick Rathbone told the club's official website.

"That is not the anterior cruciate ligament which is the big one. A lot of people tear this ligament and may not even know about it.

"However he will definitely miss Everton's last two games of the season."

Cahill was a key part of Australia's World Cup qualification success over Uruguay in November.

He was certain to be selected in coach Guus Hiddink's final squad for the tournament in Germany, where Australia has been grouped with Japan, Croatia and current world champion Brazil.

But Rathbone has confirmed Cahill faces a massive task to regain full fitness before the World Cup.

"As regards the World Cup he will obviously be a big doubt and will be receiving intensive therapy," he said.

"I have looked at the video 50 times and we know for certain he got a bang right in the back of the knee by Johnson's hip when he was in mid-air.

"He seems to land normally so there is a bit of a mystery surrounding the injury, but undoubtedly he sustained the injury there and then.

"The timescale for recovery of this injury can range from people suffering damage and never even knowing about it because it can be very minor, to the complete ruptures that require a complex operation.

"A partial tear, you would expect the consultant to be talking about a six-week period.

"At this stage and over the next couple of weeks it is a need for intensive physiotherapy."

Cahill signed with Everton for the start of the 2004-05 season following a seven-year spell with Millwall, which included an appearance in the club's 2004 FA Cup final defeat to Manchester United.

He was an instant hit with the Toffees under coach David Moyes, scoring a club-best 11 goals en route to Everton's player of the year award.

This season he has produced six goals in 32 Premiership appearances and would be sorely missed if he was unable to line-up in Germany.

Compatriot Mark Viduka remains a 50-50 prospect of playing in Middlesbrough's UEFA Cup semi-final second leg against Steau Bucharest as he battles a thigh strain.

Boro teammate Mark Schwarzer (cheekbone) and fellow Socceroo Harry Kewell (groin) are also carrying injuries, but at this stage are expected to be fit for the World Cup.

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