Ronald Koeman has stressed Everton need to react in Thursday's Europa League clash with Atalanta following their poor display against Tottenham last weekend. The Dutchman feels his side "didn't play with the right aggression" in the 3-0 home defeat to Spurs on Saturday.Koeman is glad the opportunity to bounce back has arrived swiftly, and the Everton boss is confident his players will produce a positive response against their Serie A opponents.Speaking ahead of the Group E encounter in Reggio Emilia, Koeman said: "After last weekend, we need to react."In that second half (which began with Tottenham scoring their third in the opening few seconds) we lost a lot of belief and confidence. I think we didn't play with the right aggression."The second half is not what we expect from ourselves."If you lose the game, if you are not happy with the performance, and you get an opportunity in the next few days, you like to show it."For that, I am really confident about tomorrow."