West Ham's under-21 side lost 5-3 at home to Athletic Bilbao.
West Ham's development squad lost 5-3 to their Athletic Bilbao counterparts in the first game of an international under-21 cup.
The Hammers are one of 16 teams to participate in the tournament, but the development squad manager Nick Hancock believes they were taught a football lesson by the Spaniards.
He told West Ham's website: "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."
West Ham's goals were scored by Matthias Fanimo, Elliott Lee, and Kieran Bywater.
Hancock was not making the comments to be overly critical, simply that he felt his young side were capable of a lot more, and will have to wise up if they are to get results against Europe's best.
Playing against such sides is largely a new experience for the youngsters, and with practice, comes adjustment, in terms of tactics, cutting out mistakes, and handling the occasion of such a match.
Sam Allardyce, who was watching from the stands, will hope his first team side do not commit such similar errors when they take on Burnley this coming weekend.