Hartson feels they are feeling the absence of the England centre-forward.
He told West Ham TV: "He brings that big presence up front, he's got the big number nine on his back, he's very big and strong, he's got good feet for a big man and he's aggressive in the air.
"I saw one or two balls go in the box against Swansea and I'm nudging my little son and I'm thinking if big Andy was on the pitch he gets a one, two, three and he goes and attacks the ball.
"When he's in form he's similar to Duncan Ferguson in the fact that when he gets a one, two, three on a defender and gets a run he's very, very difficult to stop. They obviously miss him, they paid a lot of money for him but he's been very unfortunate with injuries and there's nothing anybody can do about that."
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