Louis van Gaal believes a lack of composure cost Manchester United victory at Stamford Bridge as Diego Costa's stoppage-time strike earned Chelsea a 1-1 draw on Super Sunday.
Jesse Lingard opened the scoring in the 61st minute and looked to have set United on course for their fourth victory in six Premier League matches.
However, Costa pounced a minute into injury-time to snatch a point for Chelsea, leaving Van Gaal to rue his side's failure to hold on to their slender lead.
"We played a superb game until the last quarter and then we didn't keep the ball, we didn't play the attack the way we have to do," he told Sky Sports.
"We get a goal against, it is unbelievable that we don't reward ourselves. Chelsea were pushing with long balls and with more aggression, but nevertheless we have to keep the ball and you have to be composed when you defend, so you are not kicking the ball away without any meaning.
"There were a lot of times when I felt we could control the ball and play the ball. Also we could have scored after the first goal, but you don't reward yourself when you play such a game against such a fantastic team like Chelsea, you have to reward yourself.
"I am deeply frustrated because we were the better team and we didn't win, again. We had the chances and we didn't win."
Six minutes of additional time were played at the end of the second-half after Chelsea defender Kurt Zouma suffered a serious-looking knee injury in 55th minute that saw him receive lengthy treatment on the pitch before being taken off on a stretcher.
At the final whistle, a visibly angry Van Gaal was seen remonstrating with fourth official Craig Pawson and the Dutchman revealed he was angered by the timing in which referee Michael Oliver brought the match to an end.
"It was for the second time, the time was already over the six minutes and then he let them [Chelsea] kick the corner," Van Gaal added.
"We then had a free pitch for us, a lot of space and then he whistles. He did it also in the first half, maybe you remember that?
"I don't think you can do that, otherwise you don't let them kick the corner because the six minutes has passed. I don't think that is fair to the two teams."