David Moyes warned Republic of Ireland defender Declan Rice that if he doesn't learn quickly from his costly mistake at Arsenal, then he "won't progress".
West Ham were on their way to picking up a valuable point at the Emirates before Rice ducked to allow Aaron Ramsey's cross from the left to bounce past a wrong-footed Joe Hart with nine minutes to go.
The Gunners kicked on to win 4-1, with Rice afterwards holding his hands up.
"I'm young... I'll make mistakes... it's all a learning process to get to the top," he tweeted. "Thank you too (sic) the fans again who were brilliant, ready to bounce back next week."
The 19-year-old has a had a fine debut campaign with the Hammers first team, but a frustrated Moyes pulled no punches in his assessment of the mistake.
"Up until 81, 82 minutes you would have said we could have been well worthy of a point. We’d had our moments," he said.
"Look we’ve all been young players. He makes a really poor decision for the second goal. I hope he learns from it because he’s got big potential, but it really flattened the team.
"I don’t think there was (a breakdown in communication). Anyway if there was, if you’re a centre-back - and I’ve been one - if the ball’s getting played into the box the last thing you do is let it go.
"Talk about it after, head it clear. What you do is you head the ball clear and it doesn’t really matter. It’s not as if it was a massive jump. Why you would duck and leave it?"
It was a harsh lesson for Rice, and a rare black mark on what's been a year of personal highs.
Having broken into the West Ham first team, he has established himself as a huge talent, and made his international debut in Turkey last month to widespread acclaim.
Versatile and composed, Rice looks destined for a long career at the top, but Moyes said he needs to cut out major errors to improve further.
"The young boy has played great in the last few games and that’s what young players do. I read Arsene Wenger’s programme notes about putting young players in the team and how difficult it can be in the Premier League.
"I happen to think that playing young centre-backs is probably the hardest in the pitch. Declan has done a really good job and today could be a learning curve.
"He’ll have to show that it won’t happen again because if it does you don’t progress. You have to show you’re going to step up."
