His troubles did not ease during Louis van Gaal's ill-fated reign, but the 28-year-old has been given a fresh lease of life under Mourinho, playing every minute of United's three Premier League games so far this season.
"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," Fellaini told Sky Sports.
"He has given me confidence and I will give everything. (Mourinho) has had a big impact on me because when someone trusts you 200 percent you have to repay that."
Fellaini added that the team were fired up for the derby on Saturday, a game that will also see Mourinho renew his touchline rivalry with old foe Pep Guardiola in the opposition dugout.
"Everyone wants to play these games, it's a big game, a derby and everyone in the world will watch this game so it is an important game for both teams," Fellaini said.
"When you play against Liverpool and City the tension is there but I think it is very big this season because there are new managers and players.
"There will be a lot of challenges, the crowd will be behind us so it will be a good game."
(Reporting by Simon Jennings in Bengaluru; editing by Sudipto Ganguly)
