Declan Rice proved West Ham's matchwinner on the day.
Koscielny said: "We came here to win because for us the three points was very important, and next week as well to play against Chelsea. We tried to play, we did well in the first half but didn't have a lot of chances in the final third. We need to keep calmer in the final third and find the final ball, but in the second half we didn't start well and conceded early.
"After that we tried to push for the rest of the game, we had some chances and we didn't score so it is difficult to come back with no points but we need two continue to fight to make the top four.
"We are all together, it is not down to] one or two players, we are a group and we need to stay together and to push more for the next game."