The Belgian midfielder was being linked with an Old Trafford exit during the summer transfer window but has bounced back into Mourinho's plans by playing every minute of all four of the fixtures so far this season.
Fellaini was often a scapegoat for any of United's subpar performances under Louis van Gaal last term but feels now that he has the absolute full faith of the current manager.
"Louis van Gaal trusted me, I played a lot of games. This season, though, I'm playing in my position and I feel the trust of the manager," he told Sky Sports.
"He has given me confidence and I will give everything. [Mourinho's faith] has had a big impact on me because when someone trusts you 200 per cent you have to repay that.
"The manager knows my qualities, he knows what I can do.
"He spoke to me on the first day when I arrived back from holiday and I have worked hard like I do every time.
"I don't need to explain myself, all the managers I've had know my quality. I'm here today and I'm trying hard like I always do to give my best on the pitch."