MIDDLESBROUGH goalkeeper Mark Schwarzer has undergone an operation on the hernia problem which has plagued him since returning from international duty.

Earlier this week it was thought the Aussie would be fit to face Manchester United at the Riverside tomorrow.

However, Boro decided to book the 29-year-old in for an early appointment in London yesterday, in the hope they will have him playing again early in the new year.

And McClaren, who thought he would have to decide between Mark Crossley and Schwarzer in goal against the champions, admitted: "It is a blow.

"He came back with the injury and that was a disappointment, but we felt it was better to let him have surgery now.

"The alternative time would be in January or February and doing it this way, we would hope to have him back in early January.

"Hopefully it's only going to be a small operation and he may only be out for four or five weeks."

Crossley hasn't been beaten in 290 minutes since filling in while Schwarzer tried to help the Socceroos qualify for the World Cup.

The Welshman, who himself has recovered from a head injury, will line-up against United, and teammate Ugo Ehiogu knows the clash will be a tough one.

The Red Devils smashed five past Derby County on Wednesday as the revival started after a turbulent period.

And Ehiogu believes Boro are going to have to be on their guard in every department across the pitch.

"They have matchwinners in midfield and attack," said the 29-year-old, whose displays alongside Gareth Southgate have earned him England recognition.

"But it's something we have to be looking forward to. Hopefully we can restrict them to a few chances and take chances at the other end."

The clash with United follows hot on the heels of last Saturday's 2-0 defeat to League leaders Liverpool.

And after Sir Alex Ferguson brings his men to Teesside, Boro face Fulham seven days later, Newcastle on Boxing Day, Arsenal, and then on New Year's Day Everton.

And Ehiogu is convinced if McClaren has a full squad to choose from, Boro can compete with any team.

"They don't come any harder do they?" he said.

"But if we can keep everybody fit then I fear nobody."

Boro were boosted by the return to full fitness of forward Andy Campbell earlier this week.

Pacy Campbell came through 90 minutes of a reserves outing at Blackburn Rovers on Wednesday and he is now ready to put the pressure on Alen Boksic, Szilard Nemeth and Hamilton Ricard.

The 22-year-old has not played for Boro's first team since February's 3-1 FA Cup defeat to Wimbledon, and ended last season on loan to Bolton.

There has been mounting speculation that new Birmingham City manager, Steve Bruce, is set to bid for Campbell.

Argentinian midfielder Carlos Marinelli, meanwhile, is struggling to shrug off the ankle injury which kept him out of last Saturday's trip to Liverpool.

The 19-year-old's creativity was sorely missed at Anfield and McClaren would ideally like him to be available for the visit of United.

Read more about the Boro here.