Van Nistelrooy travelled to the United States this week to see leading specialist Dr Richard Steadman about his injured right knee and yesterday underwent an exploratory arthroscopic operation to determine the extent of the problem.

The former Holland international had hoped to be absent for only a couple of months, but the results of those tests revealed the knee damage was more serious than first thought and Madrid confirmed today that Van Nistelrooy could be out of action until next August.

Madrid said in a statement on their website: "It was determined that Ruud van Nistelrooy had a partially torn meniscus in his right knee. Real Madrid's medical staff, Dr Steadman and the player decided to surgically repair the damaged knee, by which he will miss between six and nine months."

The knee is the same one that Steadman operated on in 2000 when Van Nistelrooy suffered ligament damage while at PSV Eindhoven.

Van Nistelrooy's most recent appearance for Madrid came in last week's 2-0 home defeat by Juventus in the Champions League, when he was substituted nine minutes from the end.

Losing the former Holland international is a further blow for Madrid and their coach Bernd Schuster, who has found himself under increasing pressure following some poor recent results.

There have even been suggestions in the Spanish press that Schuster could lose his job this weekend if Madrid do not win their Primera Liga match at Real Valladolid.

Madrid striker Higuain, who is likely to fill Van Nistelrooy's place in the forward line, admits the prolific Dutchman will be sorely missed, both on and off the field.

"It's very sad because Ruud is a very important player for the team. He has my support and that of all the team," said the Argentinian, who scored all four of Madrid's goals in Saturday's 4-3 home win over Malaga.

"It's a great shame that he has suffered a serious injury because he is a great person who always helps you."

Higuain also preferred not to contemplate the possibility that this latest knee injury would herald the end of Van Nistelrooy's career.

"I hope it will not be like that, because he is a great striker. We hope that he is back with us as soon as possible, because as well as a great player he is a very good person," Higuain added.

The 32-year-old Van Nistelrooy joined Madrid in 2006 from Manchester United and in his first season at the Bernabeu netted 25 league goals to finish as the Primera Liga's leading scorer.

Madrid won the title that year, and they successfully defended it last season when Van Nistelrooy, despite sitting out part of the campaign with an ankle problem, netted 16 more goals, while this season he had already netted seven times in all competitions.

With Madrid already short of top-class cover for Van Nistelrooy, today's news about the striker's injury has sparked speculation about who the Spanish champions might try to sign as a replacement.

South America appears to be a likely hunting ground according to Marca, who believe Guilherme (Cruzeiro), Dentinho (Corinthians) and Alex Mineiro (Palmeiras) could be possible options.

CSKA Moscow starlet Alan Dzagoev has also been included in the list, as has Manchester United forward Carlos Tevez, who has been linked with Madrid more than once with the Argentinian's two-year loan deal at Old Trafford expiring next summer and a permanent contract yet to be signed.