Sam Allardyce has hailed summer signing Diafra Sakho's instant impact at West Ham United, but admits that he is concerned by the striker's injury.

The Hammers continued their impressive start to the season by defeating champions Manchester City 2-1 at Upton Park on Saturday to stay in the top four of the Premier League.

Sakho's 75th-minute header - his sixth top-flight goal for the club in as may starts- proved to be the winner, although the Senegalese forward was withdrawn late on with a shoulder injury.

"[Enner] Valencia and Sakho - I don't think they really know what they have done yet," the London Evening Standard quotes Allardyce as saying. "I don't think they really know how good they have been.

"They have just come here, they want to play football and I don't think they realise what such a big impact they have made and the rest of the team has been lifted by those two. We have goalscorers, we have players who believe they are going to score - none more than Sakho who has scored yet again today."

"We are four wins out of five and three wins on the trot. But for me my biggest worry is that my leading goalscorer today came off with a shoulder problem towards the end and I sincerely hope that the damage is not severe and he recovers very quickly."

West Ham return to action next weekend when they travel to the Britannia Stadium to face Stoke City.