The 28-year-old Dutch striker, United's top scorer last season, will undergo surgery next week in a bid to cure the problem.

He played for his country at Euro 2004, scoring four goals, but was banned for two international matches by UEFA for insulting the referee after the Netherlands lost 2-1 to hosts Portugal in the semi-final.

Van Nistelrooy has only played 45 minutes of the pre-season campaign so far and his injury means that as well as missing the key opening weekend trip to Chelsea, he will also skip the crucial Champions League double-header with Dinamo Bucharest next week.

Meanwhile, United found themselves 300,000 pounds (550,000 dollars) out of pocket on Thursday when they refunded all 15,000 supporters who bought tickets for the abandoned friendly encounter with Japanese side Urawa Red Diamonds.

The Old Trafford game was cancelled due to a series of electrical storms which swept across northern England.