Thomas Tuchel says it was "simply impossible" for Chelsea to allow Callum Hudson-Odoi to leave in the final day of the transfer window.
The Premier League side blocked the 20-year-old winger from making a move to Borussia Dortmund at the end of August.
Hudson-Odoi was left on the bench for the Blues' first three matches of the season, but was called on to start in the 3-0 win against Aston Villa on Saturday.
What has been said?
Although Tuchel acknowledges the switch to Germany would have been a good opportunity for the England international, he says there was no chance he would let him leave so late in the window.
"We have spoken. It was a very easy decision for me to make on the last day of the transfer period," he told reporters.
"We cannot let a player go who is in the 18/19 man squad. He is able to play as a right wing-back, a left wing-back, in the two number 10 positions. He knows the group, we know him very well. There was no chance to say yes on the last day.
"For him personally it may have been a good opportunity but for us and our targets that we want to achieve, it was simply impossible. It was an easy one for me actually."
Will Hudson-Odoi get a chance at Chelsea?
Hudson-Odoi emerged as one of the Stamford Bridge side's most promising players when he made his debut at the age of 17.
But he has been unable to secure a place in the starting XI under Antonio Conte, Maurizio Sarri, Frank Lampard or Tuchel.
He started just 10 of the 23 Premier League matches in which he played last season and Tuchel says he still has a lot of work to do if he is to cement a spot in the first team.
"Callum has to overcome the gap between training and games," he said. "I see a stronger Callum every day in training and he needs to overcome this gap and show this kind of determination that he shows in training.
"If he gets minutes, it's never a gift. Nobody gets gifts because we take maybe sometimes hard decisions like for Callum not to let him go. He needs to deserve it. I know it's not his favourite position but I think he can play this position and serve the team when it's necessary.
"[Against Villa] it was necessary and still he can do better. Still he can be more aggressive, still he needs to push himself absolutely to the limits.
"This is what he does in training. He needs to overcome this gap to fight for his place in the squad. This is life at Chelsea."