Fellaini has been at the centre of criticism following United's disastrous Premier League season, having joined former manager David Moyes at the club in a deal worth £27.5 million from Everton.
Injuries have plagued the towering midfielder since arriving in Manchester but he is determined to justify his hefty price tag next season, though a lot depends on the club's new manager, with Dutchman Louis van Gaal expected to take charge.
"I will look to show what I can do next season," the Belgium international told UK newspaper the Telegraph ahead of United's final game of the season against Southampton.
"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 said.
"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."
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