The Hammers missed the chance to go top of the Championship when they lost 1-0 at leaders Southampton on Tuesday, instead slipping five points behind Nigel Adkins' side.
And they could find themselves outside the play-off spots by the time they kick off at the Amex Stadium.
"Perhaps it wasn't our day and it is a big disappointment to lose to Southampton," said Allardyce.
"On the balance of play we should have still only been two points behind them. When we wanted it right at the most important time against someone in direct competition it hasn't happened.
"We are second in the table but still have a lot of ground to catch up. We will try and do that as quickly as we can - starting when we go to Brighton next Monday."