West Ham assistant manager Nikola Jurcevic says he is 'not satisfied' after the Europa League qualifying defeat to NK Domzale.
The former Croatia international was in charge for the clash in Slovenia as boss Slaven Bilic served a two-game UEFA touchline ban.
Matic Crnic scored twice to give the hosts a narrow 2-1 advantage in the third-qualifying round first-leg tie, but Mark Noble's 18th minute penalty did snatch a vital away goal.
"First I want to say well done to Domzale because they won today and they were really good," Jurcevic told the club's official website following the game.
"Our performance was not so good, and maybe I can say it was average in the first half and it was 1-1. We were not satisfied with this result and in the second half was played bad.
"OK, we were maybe not in our top form because it was our first official game and we were without some very important players and only had young players on the bench, but this is not an excuse, I must say."
The Hammers will play the second leg at the Olympic Stadium on Thursday - their first game in their new home - and Jurcevic is hoping for a better showing to christen the ground.
"Domzale were better and I hope and I am sure we will be better next week. It will be very difficult but I'm optimistic and I hope it will be a different game in the Olympic Stadium. It's a special game because it's the first game at the Olympic Stadium with about 54,000 spectators.
"We must be better, we must win this game in our stadium because we also have big ambitions in Europe... We must improve next week, with more players back in the squad, and I hope we can win the first game at the Olympic Stadium."
West Ham co-chairman David Gold also took to Twitter following the defeat, and admitted that the result came as a surpirse.
He wrote: "Hugely disappointed. I didn't expect to get beat tonight. The second leg is now a massive game."