"Of course, after the weekend we need to react," Koeman said ahead of the Toffees' first European fixture since 2015.
"The second half was not what we expect from ourselves.
"But the nicest part of football is that if you lose a game, if you are not happy with the performance, you get another opportunity in a few days and can show your reaction.
"We will try to dominate the game... our intention is to play offensive football, to create opportunities to score - and we need to counter the strengths of Atalanta."
Koeman, who has steered Everton past Slovakia's Ruzomberok and Croatia's Hajduk Split in qualifying, said the club must prepare for a long journey in Europe's second-tier competition after a three-year absence.
"For every football club it is really important to be part of Europe," he said.
"Let’s hope for a good game... and that the journey will take us a long way in European football."
Everton will also face Cypriot side Apollon Limassol and French side Lyon in Group E, and Koeman said he was clear about his team's approach to the competition.
"If you start a competition you want to win it. But it is a long way to go ... you have to take it step by step.
"You can rotate for several reasons but, like always, we will start with a strong team."
Everton have lost their past three games in Italy while Atalanta, who are back in European competition after 26 years, are unbeaten in five European home clashes.