Fellaini was David Moyes’ first signing after Sir Alex Ferguson left the club in the summer of 2013, but had a season to forget as United finished seventh in the Premier League.
The £27.5million signing played just 16 times in the top-flight, and was linked with several clubs last summer at a cut price.
But under Louis van Gaal, the Belgium international has found confidence and is now a major part of United’s style of play, and Fellaini says he never lost his ability, only his belief.
In an exclusive interview with Sky Sports, Fellaini said: “I’m happy. I have to continue like that, to show what I can do for my team-mates. For me, that’s the most important thing.
“[Joining United] was new for me. It was difficult. A lot of people criticised me, but people they forget quickly I played five great seasons and one year.
“Some people said I lost my football. It’s not that. OK, I lost a bit of my confidence… it was not a great season for me, but it’s good for the future because I learned a lot about that.”
Speaking ahead of his return to former club Everton on Super Sunday, Fellaini says his return to the squad in the 2-2 draw at West Brom in October gave him a new lease of life.
Fellaini scored moments after being brought on at half-time in the Hawthorns clash, and from then on he has been one of the key contributors to United’s upturn in form.
“I came back in the squad and the manager said to me ‘you have to show’. After that, my confidence grew and I showed my quality,” added Fellaini.
“For me, the team have to press well. We have to be available from the goal kick, to be in the box and I must win my physical challenges.”