The Belgian has been the subject of much criticism during the Red Devils' disastrous Premier League season, having joined former manager David Moyes at the club in a deal worth €30 million from Everton on summer transfer deadline day.
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"I will look to show what I can do next season," Fellaini told The Daily Telegraph. "I need to be fit to be good. Even when I am injured I can play but the fitness is not the same.
"I was injured for three months and that was a difficult moment for me. When you arrive in a new team you want to play, you want to stay with the team and, when you can't play, it is difficult to see the team – even off the pitch.
"When this season finishes I will start with my preparation for the next year and it will be good."
Fellaini was forced to endure an uncertain transfer window in August, with his move to United not completed until the final moments on deadline day.
"It was difficult because, for two months, I did the preparation with Everton and I didn't know if I would go or not," he recalled, "but, in the end, I wanted my transfer and I signed for Manchester United. It was a lot of stress but good in the end."