The Red Devils winger - who made a sensational move up north from Arsenal in 2018 - is currently on loan at Inter Milan after falling out of favour with the Premier League club, and he blames his former manager's regular rotation with his downturn in fortunes.
He told the BBC: "I say things as they are and in my opinion. Mourinho is one of the best coaches in the world in the way that he coaches, how he studies videos, the way he goes about things.
"But then, within the group there was that feeling that you were in the team then out.
"Sometimes I didn't play, then I did, then I didn't and as a player you lose confidence, each player lost their confidence, do you understand?
"And then an atmosphere was created that wasn't healthy."
The Chilean star - who left on load at the start of the current season after it became clear he wasn't in Ole Gunnar Solskjaer's plans - insisted he just wanted to be allowed to play.
He added: "Sometimes I'm playing well and I score a goal and he took me off. And I'd say, 'I love football, I love football'.
"I've played football since I was five and if I have the ball taken away from me, it's as if I lose my joy. I'm like that."