Mata was sold to United by Mourinho for £37.1million when the Special One was in his second spell in charge at Chelsea.
And when Mourinho was appointed at United, it was assumed Mata would be one of the most high-profile casulaties of his arrival.
But Mata has thrived under Mourinho, who claimed the Spaniard had found his "natural habitat" at United following his move from Stamford Bridge.
And Mata has insisted there is no lingering issues between him and Mourinho following the decision to offload him from Chelsea.
"Many lies were told about me and Mourinho when he arrived at the club this summer and it bothered me that every day I had to read and listen to rumours that weren't true," said Mata. "With him I'm fine.
"You read a lot of comments on Facebook and Twitter but they just make you laugh.
"I really want to lift titles at Manchester United, the Premier League is a trophy I have never won but I believe we can do it with this group."
Mata did not make Spain's squad for Euro 2016 last summer but insisted he had put that disappointment behind him to focus on United.
"It was disappointing not to go to the European Championships this summer," said Mata. "But I'm happy to be back involved and Lopetegui is a good manager."