Manchester United striker Ruud van Nistelrooy has been ruled out for a month with a hernia, Sky Sports News reported on Thursday.

The Dutch international, who scored 20 English premier league goals last season, will undergo surgery next week and his absence is a big blow to United just before the start of the new season.

United have signed England striker Alan Smith from Leeds United but experienced Norwegian forward Ole Gunnar Solskjaer will be out of action for up to three months with a knee injury.

United manager Alex Ferguson is also without Olympic-bound pair Cristiano Ronaldo and Gabriel Heinze for the start of the campaign.

United's first premier league game is against Chelsea on August 15. They play champions Arsenal in the Community Shield on Sunday and have a Champions League third qualifying round, first leg match against Dinamo Bucharest on Wednesday.