The 27-year-old Fellaini joined United in a 27 million-pound deal from Everton in 2013 to become David Moyes's first signing as manager.
But Fellaini, along with Moyes, struggled to adapt to life at Old Trafford as United finished seventh, 22 points behind champions Manchester City.
"It was a difficult season for everyone, not just me, but the criticism was on me because I was the transfer of Moyes," Fellaini told the Daily Mail on Saturday, a day before United travel to Everton in the Premier League.
"I did some good games but even those I was criticised for. A lot of people talked about me, about my quality."
"I had played five years in England and every season I played very well for Everton. Then, for one year, I lost my football, I lost my quality, I lost everything.
"That's my opinion. And now my quality has come back. But that's not right, that's not right."
Fellaini spoke with Moyes's replacement Louis van Gaal at the start of the current campaign and agreed to fight for his place.
"I talked with the new manager and he told me I have to show him, prove to him. I accept that," Fellaini said. "I am a footballer so I like that competition.
"This is a big club and I wanted to show I can play here."
The big-haired Fellaini is now a firm fixture in a United side who have risen to third in the Premier League table and Fellaini is thriving in a role in between midfield and attack.
"It is different now," he said. "The fans, the people, they are with me, not against me. Last season was bad for everyone, but I have had the strength to move on.
"I play much better than last year, but the team play much better as well, so it has been good for us all.
"That is football. I learned a lot from that. I am stronger mentally. I always thought I was, but now more than ever."($1 = 0.6585 British Pounds)
(Reporting by Michael Hann, editing by Ed Osmond)
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