West Ham boss David Moyes says they must learn from defeat at Europa League opponents Olympiacos.
The Hammers lost 2-1 in Athens, though remain top of their group.
Moyes said: "Look, we changed a lot of players tonight, we've got the Premier League coming up and we've got a League Cup game next week and we'd won the first two games in the group, which gave us leeway to make changes tonight.
"But maybe I have to recognise that while we didn't win through the Conference League games easily, as they were all tough games, coming to Olympiacos after winning away in Freiburg, which was a good result, this wasn't a good result and wasn't a good performance.
"In our heads we certainly had a bigger picture in our heads and that was going into Sunday.
"I don't think anybody could say the team we started with wasn't a good team and wasn't a team capable of winning or performing.
"Many of them have been very good players for us over the years and still are.
"If anybody wanted to see what supporters can do to make results then they should come and look at it here, because their crowd were really powerful here.
"The crowd played a huge part tonight, they really did. We found it difficult late in the game as their players managed the game, but we got back in it.
"We had to make changes to get back in it and we did do and we certainly looked more dangerous, but we never really looked as fluent with our mobility and how we played. We just didn't seem to have that.
"We were too deep for the second goal when their boy came in to shoot, and even for the first one with letting him have the shot.
"The second one, we should have been higher up and blocking the shot ten yards higher up. The first one, I'm not sure if it's gone through Dinos's legs, but he gets a run and gets the shot away, the boy.
"They hadn't really created an awful lot and over the game I don't think they created an awful lot, but we gave them big opportunities. We have to take that on the chin and move on."