Sam Allardyce saw serial goalscorer Diafra Sakho shoot down Manchester City but fears the striker may be set for a spell on the sidelines.
The reigning Premier League champions fell to a 2-1 defeat at Upton Park as Allardyce's West Ham moved to within a point of the visitors.
Morgan Amalfitano, who opened his Hammers account in the recent win over Liverpool, grabbed his second goal since his summer move as he swept home Enner Valencia's low cross in the first half.
Sakho then headed home his sixth goal in as many Premier League starts, with goal-line technology on hand to confirm his effort beat the despairing dive of Joe Hart.
David Silva pulled one back almost immediately with a well-taken individual effort but it was Sakho's goal which proved the difference as Allardyce was full of praise for the Frenchman and his strike partner Valencia.
"Valencia and Sakho -- I don't think they really know what they have done yet," Allardyce said.
"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 goal scorers, we have players who believe they are going to score -- none more than Sakho who has scored yet again today.''
Signed from Metz during a busy transfer window, Sakho has now scored in his last seven West Ham appearances, dating back to his opening strike in the Capital One Cup defeat to Sheffield United.
He has scored against Manchester United and Liverpool, as well as the match-winner against City, but Allardyce was worried after seeing him forced off following a collision with Hart.
"We are four wins out of five and three wins on the trot," Allardyce said. "But for me my biggest worry is that my leading goal scorer 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.
"When you have got someone who has scored six in six you can ill-afford to lose them at any stage and if we lose him that is going to be a big blow."
Despite universal praise for his own side, especially a man-of-the-match display from Alex Song, Allardyce felt City's own frailties led to a fifth league win of the campaign.
"We put Manchester City's back four to task and, in my opinion, they didn't hold up," he added.
"The quality of our play in the final third is why we have won the game today.
"When you have got quality players, quality players look forward to the big games where the less quality players can get anxious, but Song led us as a team today and the rest of the lads responded.'"