The Hammers boss feels that there is not too much to choose between the two sides, but admits that Southampton's results over recent matches have highlighted their strength.
“The big difference between us and them is they know how to defend better,” Allardyce said. “We have become a more threatening attacking force but our frailty is we don’t keep clean sheets like they do.
"We’ve ended up drawing the last few games instead of winning them while they’ve been winning 1-0."
West Ham are due to play Southampton tonight and they will be hoping that their attacking talents cancel out their apparent defensive frailties.
The Hammers' form has dipped of late, but their performance against Manchester United should give them confidence that they can pull off an upset at St. Mary's.
Although West Ham were not able to claim a victory against Louis van Gaal's men, they were the better side for large portions of the match and only a late Daley Blind strike denied them all three points.
It was a goal which is sure to have hurt the team at the time, but they have the opportunity to bounce back immediately tonight and a win on the south coast would certainly revive their European hopes.