West Ham midfielder James Ward-Prowse admitted frustration after their Europa League defeat at Olympiacos.
The Hammers lost 2-1 in Athens, though remain top of their group.
Ward-Prowse said: "There's a bigger picture too in that there is a group to qualify from, but we're disappointed. It's not nice to lose two games in a row but I think it's important we show a reaction on Sunday. There is a game every few days for us and there's always an opportunity to make a difference just around the corner.
The atmosphere was fantastic and, as players, that's the sort of arena you want to play in. But I thought we gifted them two goals and they didn't really have to work too hard for them. To give any team two goals makes things difficult, especially at this level.
But the positives were we came out in the second half and showed a reaction. Ultimately it came too late but it's something to remember and we look forward to playing them again at London Stadium soon."