Marco Silva conceded Fulham "cannot switch off for one second" in the Premier League, after Danny Ings' last-gasp equaliser denied the Cottagers all three points against West Ham.
The hosts appeared on course to take the spoils in the London derby at Craven Cottage, where Raul Jimenez put them in front after 24 minutes.
However, the visitors snatched a dramatic equaliser in the fifth minute of stoppage time, when substitute Ings fired past Fulham goalkeeper Bernd Leno.
And Silva acknowledged his side's brief lapse in concentration proved costly.
"It just shows that in this Premier League, you cannot switch off for one second," he told BBC Sport. "We are aware of the quick throws, and we have to keep focus and concentration.
"A big frustration for us because, before that, it was a good performance. It was clear we were the best team on the pitch. They did not have one shot [on target] in the first half.
"We should have won this game. It was not a game where West Ham pushed us back. The goal came after we switched off. It just shows you have to keep focus until the last second."
Silva also felt Fulham should have been awarded a first-half penalty when Adama Traore was challenged by Max Kilman.
"It was a clear, clear, penalty in my opinion," he added. "It's difficult to understand why it was not a penalty in that situation. I don't even want to say anything more about it."
Meanwhile, West Ham boss paid tribute to Ings, who was making only his second Premier League appearance of the season - with both coming as a late substitute.
"It was a very 'Danny Ings' goal!", Lopetegui told West Ham's official website. "He is always agile in those kinds of actions, and he took his chance very well.
"I'm really happy for him, because he hasn't played much up until now, but he is always ready to go. He is a really good example to all of his team-mates, and it's great to see his work pay off.
"As a player, it can be hard when you don't know when the coach is going to call you. The important thing is to try and always be ready, because you are going to be needed at some point.
"Scoring in the last minute today is important, because he's shown everyone that you have to believe until the end. Even though there were things we could do better today, we kept going, and we got something out of the game."
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