The England international has been a regular starter in the heart of Mourinho's defence this term and penned a new long-term deal at the club until 2024 in the summer.
However, Dier has now lifted the lid on his troublesome times at Spurs which affected his England career last season, and the 26-year-old claimed that being given a chance at centre-back was the turning point for him at Tottenham.
Speaking to the Daily Mail, Dier said: "That (leaving) was definitely on my mind. I didn't feel like I was achieving what I wanted. When your contract starts to come to an end, you question if you're part of the future.
"I had no desire to be at a club in the last year of my contract. The club knew the summer would see a decision made either way.
"I had played Pochettino's last two games at centre back, so it was difficult to go back to playing in midfield again. I explained that to Mourinho. I said to him: 'This is where I want to play. In defence'. But I knew it was a risk.
"After Chelsea he came to me. He just said: "Ok if you want to play centre back, now is your chance. Show me." I thrived off that challenge.
"It was what I had wanted to hear. It was an opportunity. My main focus is always to be in a position competing to play. Feeling there's a plan in place for me is important, too. They were the major factors in me deciding to stay here and I couldn't be happier."
Dier has scored 11 goals in 253 appearances for Tottenham since arriving from Sporting Lisbon in 2014. body check tags ::