Slaven Bilic warned West Ham to cut out the mistakes or face being punished as they were in Friday's 3-0 pre-season loss to Manchester City.
Goals from Gabriel Jesus, Sergio Aguero and Raheem Sterling saw Pep Guardiola's side to a comfortable win in Iceland, where the Hammers were caught cold.
It was a disjointed display from the London club, who were missing some players through injury, and manager Bilic conceded they fell a long way short of the level of their opponents.
"The difference is that the pace and quality that they have is scary, and you could see that," said the Croatian.
"In the second half, we had some moments in the game which were good and some opportunities but not goalscoring ones, apart from a couple.
"When we took the ball from them or when they lost possession, which was not very often, we could have opened them up because they play very high, and one proper and not very difficult ball, we could have caused them problems, but we didn't and there was the difference.
"When we made a mistake, as we did before the first goal, they punished us straight away, either with a goal or with a big chance.
"So I'm not overwhelmed and I'm not disappointed because it is part of pre-season and we will not only play against clubs like this in the Premier League.
"That said, you can't make mistakes like this, and to be fair we were making them last year, like when we played Man City at home when their first goal was similar. After they score, it is then a big problem for you to turn the game around.
"When you don't do those kind of mistakes, like for example against Spurs [in May], then you have a chance. We did the mistake very early in the game and then it's very difficult."
In better news for the Hammers, Bilic confirmed Marko Arnautovic had merely suffered a cramp before being substituted in the second half.
West Ham kick off the new campaign at Manchester United on August 13.