The loss of England full-back Glen Johnson for "a month at least" with a knee injury means Rafael Benitez has a difficult balancing act in team selection against the Coca-Cola Championship side.

Liverpool dare not suffer another blow to their season by losing in Berkshire, but Benitez knows he must protect his key players ahead of their next league game at home to Spurs on January 10.

That match, against a side harbouring top-four ambitions of their own, could have a significant impact on Liverpool's bid to force their way back into the Champions League qualification places.

It suggests that Benitez will not make wholesale changes for the cup tie and use experienced central defender Carragher at full-back instead of youngster Stephen Darby.

Carragher has filled the right-back role several times this season with Johnson suffering a string of injuries, with Martin Skrtel likely to play alongside Daniel Agger in central defence.

Johnson visited a specialist yesterday after being diagnosed with a torn medial ligament in his right knee following Tuesday's 1-0 win at Aston Villa.

Liverpool will seek a second opinion before deciding on a timescale for Johnson's rehabilitation, but there are still fears he could be out for at least two months.

England do not play again until a March friendly with Egypt, and Johnson should be at least well on the way to recovery by then.

But he will miss key Liverpool games in the FA Cup, Barclays Premier League and Europa League.

Benitez said: "Johnson has a problem, so he'll see a specialist and then we'll talk but we know he'll be out for at least one month.

"But we are waiting for another opinion and then we'll know how long."

Liverpool will still be without the suspended Javier Mascherano at Reading and Benitez could well rest Fernando Torres with the Spaniard still short of full fitness following ankle and hernia problems.