Reports have linked Dier with a move to Old Trafford since January, and those only intensified when Dier was spotted speaking to Mourinho in Portuguese after United's win over Spurs at Old Trafford last season.
And the England international has revealed that he was subsequently called into Spurs boss Mauricio Pochettino's office, with the Argentine unhappy with the situation.
"We were talking for a very long time," he told The Sunday Times. "And all of that week, really, we were in constant dialogue. It was a difficult week for me.
"The manager’s point of view was [the friendliness with Mourinho] was disrespectful to him.
"[The dialogue] was more him getting that message across than me getting my message across."
"I grew up in Portugal. Portuguese people are very proud and it’s a small country. It doesn’t have the economic power of a country such as England, so people like Mourinho, Ronaldo, they’re extremely proud of it.
"Growing up there I was within all that, you know? I didn’t want to be disrespectful to anyone.
"I completely understand where the manager was coming from but I believe I was stuck in the middle of something I couldn’t really affect."