Son Heung-min was determined to move back to Germany during the summer window.
The Tottenham Hotspur forward represented South Korea at the 2016 Olympic Games before returning to London amid transfer speculation.
"For him last season was tough for him. He came to see me and said 'I need your help'," Pochettino said after Son shone in Sunday's 1-0 win over Sunderland.
"I said, 'okay, always my door is open' and we decide to stay. He is a quiet person, nice person and then you need to fight for your place.
"But it's true from the beginning after the Olympic Games his idea was to leave to go to Germany. One thing is what the player wants, the other is the club decision.
"In his mind, he would like to move, but in the end he accepted to stay and fight for his position."