Development Squad manager Nick Haycock said his team must learn quickly from their mistakes after they slipped to a 5-3 home defeat to Spanish side Athletic Club Bilbao.
Defensive errors cost the Dev Squad dearly, particularly in the first half as they fell 3-1 behind at half-time. The Hammers rallied in the second half with goals from Elliot Lee and Kieran Bywater keeping them in it yet the Spaniards ran out deserved winners.
Speaking after the game, Haycock said: "I'm disappointed, I've just said to the boys in the dressing room if you score three goals that usually wins you a football match. But tonight we needed to score six and it doesn't happen at this level.
"We don't want to take anything away from Bilbao Athletic because they are a good side - a B team playing in the Spanish league. They're more experienced than we were but that's what the development process is about. It's about challenging young players at higher levels and seeing if they can cope."
With regular U21 players such as Reece Burke and Reece Oxford unavailable, Haycock fielded a youthful side with five U18s in the squad.
"Obviously we've made five changes and given some their debut at this level. Some of the boys have been doing well for Steve in the U18s. I'm not sure some of the experienced players did enough tonight. If you look at the manner of the goals we conceded it was football suicide.
"It was a learning curve for them. I thought four of the five goals were easily avoidable and I think we have to defend better as a team. The young lads coming in will learn that you get punished when you lose the ball high up the field.
"They'll also learn that if they make sloppy errors or don't concentrate for 95 minutes, the players at this level will punish you. The next step for every player out there tonight is pushing into the first team of the club they're at. Consistency and reliability is what's needed as a professional footballer first and foremost. If they've got flashes of brilliance you take that as an added."
First team manager Sam Allardyce was watching on, as well as assistant manager Neil McDonald and Academy manager Terry Westley.
Haycock continued to highlight the positives of his team's display: "I think the team showed fight, we got back in it at 3-2 down and had some control of the game.
"We need to be able to do that from the start of games, not just after we've scored. Any manager will tell you that football's about good players. And good players have to take what the coaches have said onto the pitch with them.
"Equally, if things aren't quite working, the bright players need to tuck each other into position. When Kevin Nolan played against Sunderland, one thing he did was tell the players around him where they should be. And I think we need a bit more of that in the team. As they're young, it makes it slightly more difficult, but that's the demands we're placing on them."
The Development Squad's next game is another tough match against Manchester United on Friday 24 October at Rush Green.
"It'll be a tough game. Under Louis van Gaal they've played some strong U21 sides. He's also given some young players a chance in the first team.
"They'll be experienced and strong. We've qualified to be in this league, we've qualified to be in this European tournament. These are the games that we want to play in. At the end of the day, yes we're disappointed with the defeat, but we have to look at it as a learning curve.
"There was good things and bad things tonight. We'll take the good things and learn from the bad, to make sure we get rid of them as soon as possible."