Manchester United [ Images ] striker Ruud van Nistelrooy [ Images ] 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 [ Images ] 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 [ Images ] is also without Olympic-bound pair Cristiano Ronaldo [ Images ] and Gabriel Heinze for the start of the campaign.

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