West Ham boss Slaven Bilic says they will not soon forget their 4-0 opening-day defeat to Manchester United.

Jose Mourinho's side delivered a commanding performance at Old Trafford on Sunday, with Romelu Lukaku scoring twice on his home debut before late goals from Anthony Martial and Paul Pogba.

The visitors were second-best throughout the contest and managed only one shot on target, while a looping Marko Arnautovic header that clipped the crossbar was the closest they came to a goal.

Bilic was despondent after the final whistle and admitted that confidence within the team was extremely low.

"We aren't going to forget about this game in a hurry," he said at a brief news conference.

"We need to analyse it. The first couple of goals we conceded from our mistakes. We were alright until then.

"We are very low at the moment. We need to pick ourselves up and prepare for the second game of the season against Southampton.

"It was a very bad result for us. They were much better than us, better with the ball and without the ball.

"We made big, big mistakes like the first goal. Very deep in the first half, it was promising and then we lost it."

West Ham won their final match of last season away to Burnley but did not manage a victory in any of their pre-season fixtures.

Bilic has urged his players not to allow their heads to drop too much before they take on Southampton next weekend.

"It was the first game of the season. We cannot lose confidence," he said. "It was against Man United and we conceded four goals."