Quique Sanchez Flores says it was "painful" to see Manchester United snatch a late winner against Watford on Saturday.
Memphis Depay gave Louis van Gaal's side a first-half lead before Troy Deeney's penalty three minutes from time looked set to earn a share of the spoils for the home side.
The Watford striker then accidentally bundled home Bastian Schweinsteiger's shot in the dying minutes, an own-goal which cost the hosts what would have been a valuable point.
Flores accepted his side were poor in the first half but felt they deserved more following their improved performance after the break.
"It was painful. I think we deserved a draw," he said afterwards. "In the second half we made it tough to score, in the last moments we scored and you feel that it is possible to get one point against a tough team.
"So to lose the point is very tough, very hard.
"Today we were more disappointed at half-time because we were not playing like Watford. In the second half, we tried to forget that, to play one match of 45 minutes. We created clear chances, four or five.
"Maybe today, Manchester United defended better than before. They substituted some players and pressed very well, defended high up the pitch. It's not normal in the big teams."
Flores refused to blame Deeney for the visitors' winning goal and insisted the forward's attempt to block Schweinsteiger's effort highlighted his work-rate throughout the match.
"He is very brave to take the challenge, to score the penalty in 87 minutes, but it's possible because Deeney fights everywhere on the pitch. You take very positive conclusions from that.
"Heurelho Gomes was waiting to take the ball with his hands. It's unlucky but it's football, sometimes this happens."
David de Gea made a number of saves to deny Watford in the second half, including a superb reaction stop to keep out Almen Abdi's deflected effort.
"We shot very well but they have an amazing goalkeeper," said Flores. "You have to be very effective in the last third if you want to score past De Gea.
"I go home, I don't punish myself too much. Sometimes you play some matches and you have to study a lot but not today. Everything was bad in the first half, not in the second. I'll go to watch the Clasico."
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