Chelsea boss Thomas Tuchel said he substituted Callum Hudson-Odoi just 31 minutes after the winger came on in the 1-1 draw at Southampton because he was not happy with his attitude.
Hudson-Odoi replaced Tammy Abraham at half-time when the Blues were 1-0 down.
"Maybe it is even unfair, but it was my feeling," Tuchel said.
Talking to BT Sport, he added: "I was not happy with his attitude, energy and counter-pressing.
"I took him off and we demand 100%, I feel he is not in the right shape to help us. It was a hard decision but tomorrow it is forgotten and he has all possibilities to start against Atletico Madrid [in the Champions League on Tuesday]."
Hudson-Odoi had made a positive impression on Tuchel following the German's arrival on 26 January.
The 20-year-old started four of Tuchel's first five games in charge, impressing as a winger and wing-back.
Tuchel added: "Everyone has to understand that I take decisions to win games and there is no problem that he makes mistakes and has a bad game, but we can't lack energy even 10% or 5%.
"He knows what I demand from him and when he doesn't reach this level, it's a hard decision."