Sam Allardyce said a combination of missed opportunities and a decision not to award his side a penalty had contributed to West Ham United's 1-0 FA Cup with Budweiser third-round replay defeat at Manchester United.

The Hammers fell behind to a ninth-minute Wayne Rooney goal when the England striker converted Chicarito's low cross. But, as they did in the initital tie and in the Barclays Premier League fixture at Old Trafford in November, they responded superbly to an early setback.

Unfortunately, both before and after half-time, the visitors were unable to give their 1,400 travelling fans something to celebrate as a succession of shots flew off-target. To compound West Ham's frustration, referee Phil Dowd ignored penalty claims for handball against Rafael before awarding a spot-kick to the Red Devils when Ryan Giggs' cross hit Jordan Spence's arm. Thankfully, Rooney blazed his penalty over the crossbar.

"First and foremost it was a bizarre goal that we gave away. There was no reason or need to let Chicarito in so easily. We should have been tracking him and there was no pressure on the ball from Anderson. Instead of stepping up to play offside, which we couldn't do, it became an easy and simple goal for them to take.

"Before that and after that our performance was very, very good and certainly for the 20 minutes or more of the second half, we continued to press and test Manchester United's resolve. Unfortunately, because of our incapacity to put a chance in the back of the net at the moment, we couldn't find a goal.

"At one stage in the second half, Rafael clearly handled the ball - there is no doubt about that. The referee was in the perfect position but he didn't give it. Because he didn't give it, it didn't allow us to get back into the game at that particularly time by scoring from the penalty spot. Who knows what might have happened there? We could have easily have won the game and gone through to the next round because we were the stronger side at the time.

"Then, of course, the disappointing thing happens up the other end. I know Wayne Rooney missed the penalty and it kept us in the game, it was probably justice in the end. Jordan Spence did handle the ball, but he didn't handle it any worse or any more than Rafael handled it. If you are going to give them a penalty, you have got to give us a penalty.

"The referee has no excuse for his position because I've looked on the laptop, but that's not the reason why we have lost. It has contributed, but the reason we lost is because we didn't score despite creating so many chances at Old Trafford."

The manager said there were positives to take from the defeat - notably the return of Mohamed Diame from a hamstring injury and the players' ability to perform in a new 3-5-2 formation.

"We made tactical changes with a shortage of defenders and Alou Diarra stepped into the back-three and was very, very good. Apart from the goal we conceded, Alou, James Tomkins and Winston Reid were very good defensively. We frustrated Manchester United and played with wing-backs - young lads who kept Nani, Ryan Giggs and Valencia very quiet.

"That allowed our midfield to start playing. We had two up top and tried to play off Vaz Te and Carlton Cole and it allowed us to create and get into the final third and Manchester United's box, but we couldn't find the opportunity to score. We had plenty of good chances to do that, but unfortunately couldn't."

While it may be a cliche, the reality is now that West Ham can concentrate all their efforts on the Barclays Premier League - starting with the visit of Harry Redknapp's Queens Park Rangers on Saturday.

"We need to keep our performance at this level. We had this level at Upton Park and nearly beat them there and we had a fantastic level of performance on Wednesday, but what I cannot accept was the level of performance we produced against Sunderland. We cannot have anything other than this on Saturday when we face QPR."