The Toffees were swept aside by a classy Tottenham Hotspur side at Goodison Park on Saturday.
The club travel to Italian outfit Atalanta this evening, hoping to get back to winning ways.
And Koeman is now urging his players to seize their chance for a run in Europe – and says he is grateful for the quick return to action after Saturday's 3-0 hammering.
“Of course, after the weekend we need to react,” said Koeman.
“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.
“I am really confident about tomorrow."
Everton have beaten MFK Ruzomberok of Slovakia and Croatian giants Hajduk Split to reach this stage of the competition, and Koeman admits it is “great” for the Merseyside outfit to be involved in European football.
"For every football club it is really important to be part of Europe.
"Everton was out of it for many years but we are back and it is great for the players and staff – and also for the fans.
“Let’s hope for a good game tomorrow and that the journey will take us a long way in European football.”
Everton have had a mixed start to the season, taking just four points from their opening four fixtures.
The club invested heavily over the summer to compete with the top six sides in the league.
