A 2-1 victory over London rivals Tottenham on Wednesday put the Hammers into the competition's last four for the second time in four years but the 59-year-old expects his defence to be in for a severe test.
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"The big disappointment is that we have Manchester City in the semi-final," Allardyce told West Ham's official website.
"Away from home, they haven't been too clever but, at home, they've been scoring an average of four goals a game.
"You never know, and we'll go and give it our best shot, but we want to see more performances like that and pick up more points in what is the main priority for us - the Premier League."
Both legs of the semi-final will take place in January, with West Ham travelling to the Etihad Stadium for the opener.