Manchester United were linked with a move for England intentional Dier in the summer of 2016 – but not bid was ever made.
Spurs locked their midfield star into a new contract to deter suitors – although United continued with their interest.
And Pochettino has accused Mourinho of speaking to Dier while United began “putting pressure on him” to move.
“I found out that United have made an approach and the player is being destabilised,” Pochettino wrote in the Daily Mail.
“His people have been putting pressure on him, although United are not promising anything.
“Mourinho and I had finished our interviews at Old Trafford [after the 1-0 defeat by Manchester United, December 2016] and the players were doing their warm-down on the pitch.
“When Jose was done with the press, he stood by the entrance to the tunnel and regarded the returning players.
“He greeted Moussa Sissoko and hugged Dier. They passed by me en route to the dressing rooms, laughing, speaking in Portuguese. Maybe it is a common Mourinho tactic, but he put Eric in a compromising position. You cannot do that after a defeat.”
Pochettino then spoke to Dier himself to ask about the conversation with Mourinho.
And he reckons the issue is now over.
The Tottenham boss added in the Mail: “‘Are you friends with Mourinho?’ I asked him. ‘No, but I’ve known him a long time, from my time in Portugal… one of his godsons coached me. He always says hello.’
“I sat down with Eric after lunch on Monday and we chatted for four hours about the whole shebang: his agent, family, confusion.
“As for the Mourinho incident, ‘What could I have done?’ Eric asked me.”