Sam Allardyce admitted first-half mistakes cost his West Ham United side any chance of achieving a positive result at Manchester United.
The Hammers made two fatal errors before the break, allowing Danny Welbeck and Adnan Januzaj to put the Red Devils into an unassailable 2-0 half-time lead.
While West Ham improved after the break at Old Trafford, substitute Ashley Young curled in a superb third before Carlton Cole pulled a consolation goal back late on. The visitors then had a penalty shout for handball by Tom Cleverley turned down but, in truth, David Moyes' side deserved the three points.
Read a full Match Report from Old Trafford
"They were two silly mistakes for the first and second goals, full-stop," admitted Big Sam. "You don't do that at Manchester United or else you'll get punished and punished we got.
"While we didn't those sort of errors on Wednesday night at Tottenham, we have seen them here. We played against two top sides and we've won against Tottenham because we didn't make those sorts of errors, but we've made them against Manchester United and lost the football match.
"That gave Manchester United that little bit of relief that they were looking for because they were going to be nervous if we carried on sticking to the gameplan, which was very much as it was at Tottenham. We wanted to get to half-time and then the opposition would tire and we would have had more space to exploit and would have created more chances.
"The stats tell us that Manchester United, when only drawing at half-time are won none, drawn two and lost three. We made our players very clear about that and how important it was to get to half-time because Manchester United's energy would all be at the beginning of the game.
"To relieve the pressure on us, we told them to defend correctly and we'd always have a chance in the second half. Unfortunately, because of our mistakes, we made life extremely difficult for ourselves to come back."
Manchester United's first goal came when Mo Diame gave away a free-kick inside his own half on 25 minutes. The set piece was taken quickly, allowing Wayne Rooney and Welbeck to exchange passes before the latter fired low past Barclays Premier League debutant Adrian.
Then, as half-time approached, youngster Januzaj benefitted when James Collins saw his clearance blocked. The ball was worked to Welbeck, who found the teenager to turn Collins and finish confidently past the Spaniard.
"You can't afford to make simple errors inside your own half," continued the manager. "We keep telling the players the fact that you have to give yourself enough time and space to play it the right way in your own half.
"If you volunteer possession, then one pass from opposition with this sort of quality can mean they score against you. When we do our attacking play, we talk a lot about trying to win the ball back as quickly as we can, as high up the pitch as we can, because we're going to find Manchester United even more difficult to break down than they will find breaking us down.
"To promote that in our own attacking play and then to do the opposite by giving possession to the opposition in our half, and them score from it, was the big disappointment. At the end of the day, fresh players came in and weren't as on top of their game as I expected them to be.
"After Wednesday, I thought everybody would be dying to get out there and play, but in the first half I didn't think we had the same belief that we had on Wednesday night against Tottenham."
West Ham did improve after half-time, but was that down to the Hammers themselves or Moyes' men sitting back on their two-goal advantage?
"A bit of both, really," said the manager. "I still think we should have had a penalty at 3-1, which would have made the last few minutes interesting. On the balance of what referees give these days, I still think Cleverley has handled the ball, but it didn't happen. We saw it happen here last year in the FA Cup third round replay and it happened again here, but we haven't got a penalty.
"You can only ask the referee in those positions to give what he sees and for me it was a definite penalty. That's not the crux of the matter, though, because the crux of the matter was ourselves.
"We need to get our injured players back and I'm trying to move them back into the team as quickly as possible. Our small squad is being really tested at the moment and we really need as many players fit as possible at this stage of the season."
The manager is not concerned about the Club's league position as much as he is by the haul of 14 points from 17 league games played.
"It doesn't matter if we're in the bottom three or not - it's 14 points from 17 games and that's fewer points than games played. That's got to be caught up as soon as possible and that's a massive task that will take six, seven or eight games.
"Everyone talks about winning one game after the other and us being all right but it has to happen when it hasn't happened since we got back into the Premier League. It's got to happen now. It's a long haul through Christmas and January now for us to get ourselves away from the relegation zone.
"Like I said, we'll be OK when we get everybody back - Andy Carroll, Ricardo Vaz Te, Stewart Downing and Winston Reid. All these players need to be fit and part of the squad. When they are, along with the others, we'll achieve better results and pick up more points."