The Spaniard spent five years at Old Trafford, making 132 appearances for the club.
Herrera enjoyed a good relationship with the United supporters thanks to his hard work and commitment on the pitch.
But the 30-year-old became disillusioned with the higher powers at work and the priorities of the football club.
"I was very happy at that incredible club. I am very grateful to the supporters," Herrera said (as reported in the Metro).
"Frankly, I was immensely happy in Manchester, but at the club, there were times when I felt that football was not considered the most important thing."
When prompted as to whether he thought money was more important to United's chiefs, Herrera didn't risk making accusations.
"That does not come out of my mouth," he said.
"I don’t know, but football was not the most important thing in Manchester.
"I do not want to compare, all I know is that here [at PSG], I feel like I’m breathing football on all sides. And I like it."
Herrera was previously vocal about his discontent with the United board's handling of his contract situation.
He seems to point toward there being an issue that stems from way above the manager's head.
Gary Neville has previously criticised club vice-chairman Ed Woodward - even calling for him to be sacked.
United face Brighton on Sunday in what is a must-win game for the Red Devils who sit in tenth place in the Premier League, ten points off the top four.
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