Everton fell 1-0 at Bournemouth on Saturday.
Koeman accepted the loss could "maybe" prove a dose of reality, after the Toffees could have moved top of the league with a victory and the help of results elsewhere.
"We know we will not win every game, it's the Premier League, we need to be prepared for everyone," said Koeman.
"In my opinion Bournemouth are a good team, they will not fight against relegation.
"From us it was not good enough. First half Bournemouth were the better team, more aggression, winning second balls.
"They started much better than we did. It was disappointing after 45 minutes.
"The second-half at least we showed what we need to do. Good reaction of the team, some chances, not really big opportunities but we were the more dangerous team.
"It's a pity because you need to start like that."