Sam Allardyce admitted a slow start to the second half had cost his West Ham United side dear in their 5-1 Barclays Premier League defeat at Arsenal on Wednesday.

The Hammers weathered an early Gunners storm before taking the lead through Jack Collison's 18th-minute shot - his first of the season. However, Lukas Podolski equalised within four minutes with an equally-impressive left-foot shot.

West Ham re-grouped and were only denied a half-time lead when Carlton Cole's shot was cleared off the line by Jack Wilshere. The opening ten minutes of the second half was a different story, however, as Podolski laid on three of Arsenal's four goals during an amazing blitz. Olivier Giroud scored Arsenal's second and fifth either side smart finishes from Santi Cazorla and Theo Walcott as the home side ripped the Hammers to shreds with wave after wave of lightning attacks.

The visitors' misery was compounded by the loss of young defender Dan Potts to concussion, with the teenager receiving ten minutes of treatment on the pitch before being stretchered off and taken by ambulance to hospital.

Big Sam admitted the horror show immediately after the break had undone all the hard work done before the break.

 "We nearly made it 2-1 to us when Carlton Cole nipped in behind the Arsenal and I thought he was going to get that one in the back of the net," said the manager. "He beat the keeper but they managed to clear it off the line.

"I'm just stunned by what happened at the start of the second half because we were so comfortable from a defensive point of view. Arsenal were having a huge amount of possession, as you expect from them here, but their chances were few and far between. Podiolski had to hit an unbelievable shot to score and the only other chance, Joey O'Brien cleared off the line.

"From nearly going in 2-1 up, we came out for the second half and didn't start very well. Walcott got straight down the right-hand side from the kick-off, which we didn't let him do in the entire first half, and that put us under pressure.

"Another thing happened that I never expected was them scoring from a corner. That lifted Arsenal and they got to their brilliant best and we couldn't live with them for ten minutes, and in that ten minutes they scored four goals.

"At the end of the day, when their players were flying like they were, you have got to be good in terms of your resilience at that time and we couldn't find the resilience to cope with their brilliance. They were on top form for that particular spell and were clinical in their finishing. Because they were clinical is why they scored the goals they did and won so comfortably."

One positive was the sight of Collison scoring his first goal since last season's Play-Off semi-final first-leg victory at Cardiff City in May 2012. On scoring, the No10 raced to celebrate with Head of Sports Medicine Andy Rolls, who oversaw the midfielder's return to action last month after six months on the sidelines.

"It was a very good goal, too, a left-foot volley from the edge of the box after a corner. It put us a goal in front, and it is a while since that has happened away from home but we couldn't hold on to that too long."

The manager also confirmed that Potts would stay in hospital overnight for observation after suffering his second concussion in the space of a little more than six months. The left-back also spent a night in hospital after suffering mild concussion following a clash of heads in a pre-season match at Southend United last July.

"Dan Potts has got a severe concussion. We hope that is all it is. He has gone off to hospital and will be under observation at the hospital for however long to get him round and get him home."