David Moyes has admitted than Declan Rice's mistake cost West Ham dearly in a 4-1 defeat to Arsenal at the Emirates on Sunday.
Rice ducked away from Aaron Ramsey's cross late in the second half with the scoreline still 1-1, only to watch the ball sail past a stony faced Joe Hart and into the back of the net without a touch being applied by a player from either side.
Speaking to the press after the match, Moyes said: "I think up until about 81, 82 minutes you'd say we were looking as if we could be well worthy of a point. We defended well, we had our moments in the first half where Marko got free a couple of times and you maybe fancied him to get a shot away.
"But we've all been young players, a young player makes a mistake for the second goal. I hope he learns from it, because he's got big potential but it really flattened the team."
Asked if he believed the goal was the result of a communication error between his goalkeeper and the young defender, he said: "No, I don't think there was. If you're a centre-half, and I've been one, if the ball's getting played into the six yard box then the last thing you do is let it go. You talk about it after you head it clear.
Moyes on the mix-up for the second Arsenal goal: "A young player makes a really poor mistake for the second goal. I hope he learns from it"— Chris Deeley (@ThatChris1209) April 22, 2018
"To be fair, we had done most things really well defensively. We'd tried to be inch-perfect on our defending, tried to stop Arsenal, and we knew we had players who could cause them problems if we got enough of the ball. But after the goal, I have too many attacking players on the pitch when we go 2-1 down, it means that then they get the third one and it's just too difficult to get back into it."
He also insisted that any shout from Pierre-Emerick Aubameyang shouldn't have made a difference to the situation, adding: "No but if he did, I wouldn't count that. What you do is head the ball clear, it doesn't really matter. It's not as if it was a massive jump, it's just...why you would duck and leave it...
"Look, the young boy's played great in the last couple of games, and that's what young players do. I read Arsene Wenger's notes today about putting young players in the team and what that means, and how difficult it can be in the Premier League.
"I happen to think that playing young centre-halves is probably the hardest position now on the pitch, even harder than playing a young goalkeeper, and I think Declan's done a really good job but today will be a learning curve for him and he has to show that it doesn't happen again - because if it happens again then you don't progress. You have to show that you're going to step up."