With one point from the last five Barclays Premier League matches away from home, the Hammers' form on the road is also troubling Allardyce ahead of the trip to Old Trafford. They have scored just five times away from Upton Park this term in the league.
"Away from home we haven't been very good recently," he said. "The trouble is we don't score. It's that simple. Five goals. It's not good enough.
"You don't need to look any farther than that. Because that puts your defenders under too much pressure. Then they make a mistake, and you end up losing a game you should have got something out of.
"You've got to put your chances away, and away from home we're not."