However, Valdano also admitted there was no player available comparable to Pepe, describing the Portugal international as "irreplaceable".

Pepe's injury leaves Madrid with just three recognised centre-backs in Raul Albiol, Ezequiel Garay and Christoph Metzelder, although Sergio Ramos can also play in the middle.

It could mean the capital club, having spent around 250million euros on new talent this summer, will look to bring in more defensive reinforcements next month.

Valdano said on the club's official website: "Pepe is an irreplaceable player. There is no other Pepe on the market.

"From tomorrow we will evaluate if we are going to make a signing to make us feel more comfortable and to reinforce the squad, but it is something we will talk about tomorrow because today we are concerned about the fact we have lost the player.

"We are not ruling it (a signing) out, but neither are we sure because we have to look at the market."

Pepe was stretchered off just prior to half-time in last night's Primera Division encounter with Valencia after landing awkwardly following an aerial challenge.

The Portugal international underwent an MRI scan at the Hospital Sanitas La Moraleja in Madrid this morning, where it was confirmed he has suffered a torn anterior cruciate ligament in the right knee.

Madrid confirmed in a medical statement the player would be operated on in the near future, and would be out for an estimated six months.

That would rule Pepe out of the rest of Madrid's Primera Division and Champions League campaigns and also puts into doubt his participation at next summer's World Cup.

The Spanish season is scheduled to finish on the weekend of May 15/16, while the Champions League final - to be held at Madrid's Bernabeu stadium - is on May 22, and the World Cup begins on June 11.

The Brazilian-born 26-year-old has been one of Madrid's key players since joining the Spanish giants from Porto for 30m euros in summer 2007, helping them win the league title in 2008.

The news will also be a cause concern for Portugal coach Carlos Queiroz.

Portugal, having reached next summer's World Cup in South Africa via the play-offs, were drawn in the 'Group of Death' alongside Brazil, Ivory Coast and North Korea.

Their first Group G match is against Ivory Coast on June 15.