Dier was reportedly the subject of United's interest in the summer as Mourinho scoured the market for a towering presence to patrol the centre of midfield, but, according to Pochettino's recently released book Brave New World: Inside Pochettino's Spurs, the Argentine was angered by the Portuguese's brief conversation with Dier after his side lost at Old Trafford in December.
Pochettino quizzed Dier about his relationship with the United boss but has since resolved the matter with England international. The Argentine now insists the matter has been firmly put to bed ahead of Tottenham's trip to Manchester on Saturday (28 October), with his side in need of a win in order to leapfrog United and claw back ground on league leaders Manchester City, whom they trail by five points.
"That is in the past. I don't like to speak about my book because it's in the past," Pochettino said in his press conference. "I wrote the book in different circumstances to today, which happened a year ago. The book only tried to show how difficult our job is, for every manager, not only me that has problems - every single manager."
Despite being unnerved by Mourinho's jovial chat with Dier, who has three-and-a-half years left on his current Tottenham contract, Pochettino insists his relationship with the United chief was not diminished in any way. Mourinho failed to prise Dier away from Tottenham in the summer; he eventually plumped for Nemanja Matic as the former Everton youth star remained in north London.
"No. It's a thing that is in the past and I was honest in the book," Pochettino said. "My relationship with him [Mourinho] is very good, we're in contact. That is only describing one, small situation which is not to put in the headline on the newspaper.
"There's nothing wrong, I only describe a situation that happened in the past. It's not an issue.
"It was in the summer, you know very well what happened. His dad passed away and for different circumstances, I called him, but it was a few months ago. Only one situation in public. I describe in the book what happened, only I give my opinion but it's not to find more. I am very grateful to him, when I met him at Real Madrid. Nothing changed from one situation."